Chin. Phys. B

Stability analysis of multiple-lattice self-anticipative density integration effect based on lattice hydrodynamic model in V2V environment

Geng Zhang(张埂) and Da-Dong Tian(田大东)

Chin. Phys. B, 2021, 30 (12): 120201. DOI: 10.1088/1674-1056/ac05b4

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Under the environment of vehicle-to-vehicle (V2V) communication, the traffic information on a large scale can be obtained and used to coordinate the operation of road traffic system. In this paper, a new traffic lattice hydrodynamic model is proposed which considers the influence of multiple-lattice self-anticipative density integration on traffic flow in the V2V environment. Through theoretical analysis, the linear stability condition of the new model is derived and the stable condition can be enhanced when more-preceding-lattice self-anticipative density integration effect is taken into account. The property of the unstable traffic density wave in the unstable region is also studied according to the nonlinear analysis. It is shown that the unstable traffic density wave can be described by solving the modified Korteweg-de-Vries (mKdV) equation. Finally, the simulation results demonstrate the validity of the theoretical results. Both theoretical analysis and numerical simulations demonstrate that multiple-lattice self-anticipative density integration effect can enhance the stability of traffic flow system in the V2V environment.

Prediction of epidemics dynamics on networks with partial differential equations: A case study for COVID-19 in China

Ru-Qi Li(李汝琦), Yu-Rong Song(宋玉蓉), and Guo-Ping Jiang(蒋国平)

Chin. Phys. B, 2021, 30 (12): 120202. DOI: 10.1088/1674-1056/ac2b16

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Since December 2019, the COVID-19 epidemic has repeatedly hit countries around the world due to various factors such as trade, national policies and the natural environment. To closely monitor the emergence of new COVID-19 clusters and ensure high prediction accuracy, we develop a new prediction framework for studying the spread of epidemic on networks based on partial differential equations (PDEs), which captures epidemic diffusion along the edges of a network driven by population flow data. In this paper, we focus on the effect of the population movement on the spread of COVID-19 in several cities from different geographic regions in China for describing the transmission characteristics of COVID-19. Experiment results show that the PDE model obtains relatively good prediction results compared with several typical mathematical models. Furthermore, we study the effectiveness of intervention measures, such as traffic lockdowns and social distancing, which provides a new approach for quantifying the effectiveness of the government policies toward controlling COVID-19 via the adaptive parameters of the model. To our knowledge, this work is the first attempt to apply the PDE model on networks with Baidu Migration Data for COVID-19 prediction.

Optimal control strategy for COVID-19 concerning both life and economy based on deep reinforcement learning

Wei Deng(邓为), Guoyuan Qi(齐国元), and Xinchen Yu(蔚昕晨)

Chin. Phys. B, 2021, 30 (12): 120203. DOI: 10.1088/1674-1056/ac3229

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At present, the global COVID-19 is still severe. More and more countries have experienced second or even third outbreaks. The epidemic is far from over until the vaccine is successfully developed and put on the market on a large scale. Inappropriate epidemic control strategies may bring catastrophic consequences. It is essential to maximize the epidemic restraining and to mitigate economic damage. However, the study on the optimal control strategy concerning both sides is rare, and no optimal model has been built. In this paper, the Susceptible-Infectious-Hospitalized-Recovered (SIHR) compartment model is expanded to simulate the epidemic's spread concerning isolation rate. An economic model affected by epidemic isolation measures is established. The effective reproduction number and the eigenvalues at the equilibrium point are introduced as the indicators of controllability and stability of the model and verified the effectiveness of the SIHR model. Based on the Deep Q Network (DQN), one of the deep reinforcement learning (RL) methods, the blocking policy is studied to maximize the economic output under the premise of controlling the number of infections in different stages. The epidemic control strategies given by deep RL under different learning strategies are compared for different reward coefficients. The study demonstrates that optimal policies may differ in various countries depending on disease spread and anti-economic risk ability. The results show that the more economical strategy, the less economic loss in the short term, which can save economically fragile countries from economic crises. In the second or third outbreak stage, the earlier the government adopts the control strategy, the smaller the economic loss. We recommend the method of deep RL to specify a policy which can control the epidemic while making quarantine economically viable.

A new algorithm for reconstructing the three-dimensional flow field of the oceanic mesoscale eddy

Chao Yan(颜超), Jing Feng(冯径), Ping-Lv Yang(杨平吕), and Si-Xun Huang(黄思训)

Chin. Phys. B, 2021, 30 (12): 120204. DOI: 10.1088/1674-1056/ac29af

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A new algorithm for reconstructing the three-dimensional flow field of the oceanic mesoscale eddies is proposed in this paper, based on variational method. Firstly, with the numerical differentiation Tikhonov regularizer, we reconstruct the continuous horizontal flow field on discrete grid points at each layer in the oceanic region, in terms of the horizontal flow field observations. Secondly, benefitting from the variational optimization analysis and its improvement, we reconstruct a three-dimensional flow field under the constraint of the horizontal flow and the vertical flow. The results of simulation experiments validate that the relative error of the new algorithm is lower than that of the finite difference method in the case of high grid resolution, which still holds in the case of unknown observational errors or in the absence of vertical velocity boundary conditions. Finally, using the reanalysis horizontal data sourcing from SODA and the proposed algorithm, we reconstruct three-dimensional flow field structure for the real oceanic mesoscale eddy.

Near-optimal control of a stochastic rumor spreading model with Holling II functional response function and imprecise parameters

Liang'an Huo(霍良安) and Xiaomin Chen(陈晓敏)

Chin. Phys. B, 2021, 30 (12): 120205. DOI: 10.1088/1674-1056/ac0eea

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In recent years, rumor spreading has caused widespread public panic and affected the whole social harmony and stability. Consequently, how to control the rumor spreading effectively and reduce its negative influence urgently needs people to pay much attention. In this paper, we mainly study the near-optimal control of a stochastic rumor spreading model with Holling II functional response function and imprecise parameters. Firstly, the science knowledge propagation and the refutation mechanism as the control strategies are introduced into a stochastic rumor spreading model. Then, some sufficient and necessary conditions for the near-optimal control of the stochastic rumor spreading model are discussed respectively. Finally, through some numerical simulations, the validity and availability of theoretical analysis is verified. Meanwhile, it shows the significance and effectiveness of the proposed control strategies on controlling rumor spreading, and demonstrates the influence of stochastic disturbance and imprecise parameters on the process of rumor spreading.

Three-party reference frame independent quantum key distribution protocol

Comfort Sekga and Mhlambululi Mafu

Chin. Phys. B, 2021, 30 (12): 120301. DOI: 10.1088/1674-1056/abff1f

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We present a three-party reference frame independent quantum key distribution protocol which can be implemented without any alignment of reference frames between the sender and the receiver. The protocol exploits entangled states to establish a secret key among three communicating parties. We derive the asymptotic key rate for the proposed protocol against collective attacks and perform a finite-size key security analysis against general attacks in the presence of statistical fluctuations. We investigate the impact of reference frame misalignment on the stability of our protocol, and we obtain a transmission distance of 180 km, 200 km, and 230 km for rotation of reference frames β=π/6, β=π/8 and β=0, respectively. Remarkably, our results demonstrate that our proposed protocol is not heavily affected by an increase in misalignment of reference frames as the achievable transmission distances are still comparable to the case where there is no misalignment in reference frames (when β=0). We also simulate the performance of our protocol for a fixed number of signals. Our results demonstrate that the protocol can achieve an effective key generation rate over a transmission distance of about 120 km with realistic 10⁷ finite data signals and approximately achieve 195 km with 10⁹ signals. Moreover, our proposed protocol is robust against noise in the quantum channel and achieves a threshold error rate of 22.7%.

Degenerate asymmetric quantum concatenated codes for correcting biased quantum errors

Ji-Hao Fan(樊继豪), Jun Li(李骏), Han-Wu Chen(陈汉武), and Wen-Jie Liu(刘文杰)

Chin. Phys. B, 2021, 30 (12): 120302. DOI: 10.1088/1674-1056/ac05a8

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In most practical quantum mechanical systems, quantum noise due to decoherence is highly biased towards dephasing. The quantum state suffers from phase flip noise much more seriously than from the bit flip noise. In this work, we construct new families of asymmetric quantum concatenated codes (AQCCs) to deal with such biased quantum noise. Our construction is based on a novel concatenation scheme for constructing AQCCs with large asymmetries, in which classical tensor product codes and concatenated codes are utilized to correct phase flip noise and bit flip noise, respectively. We generalize the original concatenation scheme to a more general case for better correcting degenerate errors. Moreover, we focus on constructing nonbinary AQCCs that are highly degenerate. Compared to previous literatures, AQCCs constructed in this paper show much better parameter performance than existed ones. Furthermore, we design the specific encoding circuit of the AQCCs. It is shown that our codes can be encoded more efficiently than standard quantum codes.

Quantum reflection of a Bose-Einstein condensate with a dark soliton from a step potential

Dong-Mei Wang(王冬梅), Jian-Chong Xing(邢健崇), Rong Du(杜荣), Bo Xiong(熊波), and Tao Yang(杨涛)

Chin. Phys. B, 2021, 30 (12): 120303. DOI: 10.1088/1674-1056/ac051e

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We study dynamical behaviors of a Bose-Einstein condensate (BEC) containing a dark soliton reflected from potential wells and potential barriers, respectively. The orientation angle of the dark soliton and the width of the potential change play key roles on the reflection probability R_s. Variation of the reflection probability with respect to the orientation angle θ of the dark soliton can be well described by a cosine function R_s~cos[λ(θ-π/2)], where λ is a parameter determined by the width of the potential change. There are two characteristic lengths which determine the reflection properties. The dependence of the reflection probability on the width of the potential change shows distinct characters for potential wells and potential barriers. The length of the dark soliton determines the sensitive width of potential wells, whereas for potential barriers, the decay length of the matter wave in the region of the barrier qualifies the sensitive width of the barrier. The time evolution of the density profiles of the system during the reflection process is studied to disclose the different behaviors of matter waves in the region of the potential variation.

Quantifying coherence with dynamical discord Suggested by editors

Lian-Wu Yang(杨连武) and Yun-Jie Xia(夏云杰)

Chin. Phys. B, 2021, 30 (12): 120304. DOI: 10.1088/1674-1056/ac16d1

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Quantum coherence and discord are two kinds of manifestations of nonclassicality. By calculating the coherence and discord in the specific bipartite quantum systems, we show quantitative connections between the coherence and the discord in the bipartite quantum systems created from local systems with the help of incoherent operations. We show that the coherence bounds the dynamical discord, and under particular conditions of the initial quantum states, the coherence of single systems is equal to the dynamical discord. We extend these results to the multipartite quantum systems.

Simulation of the gravitational wave frequency distribution of neutron star-black hole mergers

Jianwei Zhang(张见微), Chengmin Zhang(张承民), Di Li(李菂), Xianghan Cui(崔翔翰), Wuming Yang(杨伍明), Dehua Wang(王德华), Yiyan Yang(杨佚沿), Shaolan Bi(毕少兰), and Xianfei Zhang(张先飞)

Chin. Phys. B, 2021, 30 (12): 120401. DOI: 10.1088/1674-1056/abff28

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LIGO-Virgo has observed the gravitational waves (GWs) from the coalescence of binary black hole (BBH) and binary neutron star (BNS) during O1 and O2, and the ones from NS-BH are expected to be hunted in the operating O3 run. The population properties and mass distribution of NS-BH mergers are poorly understood now, thus researchers simulated their chirp mass ($\mathcal{M}$) distribution by a synthetic model, in which the BHs and NSs were inferred by LIGO-Virgo (O1/O2), and obtained the values in the range of $2.1M_{\odot}<\mathcal{M}<7.3M_{\odot}$. In this paper, we further simulate the GW frequency ($f_{\scriptscriptstyle {\rm GW}}$) distribution of NS-BH mergers by the above-stated synthetic model, with a basic binary system model through the Monte Carlo method. Our results predict that the median with 90% credible intervals is $165_{-64}^{+475}$~Hz in the case of Schwarzschild BH when the system just before merger, and this GW frequency is expected to increase several times in the merger stage, which is lying in the frequency band of LIGO-Virgo, i.e., about 15 ~Hz to a few kHz. Our results provide an important reference for hunting the NS-BH mergers by the on-going O3 run of LIGO-Virgo.

Merging and splitting dynamics between two bright solitons in dipolar Bose-Einstein condensates

Xin Li(李欣), Peng Gao(高鹏), Zhan-Ying Yang(杨战营), and Wen-Li Yang(杨文力)

Chin. Phys. B, 2021, 30 (12): 120501. DOI: 10.1088/1674-1056/abff3f

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We numerically study the interaction dynamics of two bright solitons with zero initial velocities in the one-dimensional dipolar Bose-Einstein condensates. Under different dipolar strengths, the two bright solitons can merge into a breathing wave, and then split or propagate constantly after several oscillating periods. We quantitatively study the breathing frequency of wave after merging and the asymmetry property of solitons after splitting, and analyze their formation mechanism by the system's energy evolution. Also, the change of initial phase difference brings distinct effects on the soliton interaction. Our results provide insight into the new dynamical phenomena in dipolar systems and enrich the understanding for interaction between dipolar solitons.

Heterogeneous dual memristive circuit: Multistability, symmetry, and FPGA implementation Suggested by editors

Yi-Zi Cheng(承亦梓), Fu-Hong Min(闵富红), Zhi Rui(芮智), and Lei Zhang(张雷)

Chin. Phys. B, 2021, 30 (12): 120502. DOI: 10.1088/1674-1056/abfa03

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An improved heterogeneous dual memristive circuit (DMC) is proposed based on Chua's circuit, which shows good symmetry and multistablility. For the difficulty in controlling the initial conditions, which restricts the engineering applications, the 3rd-order model (3OM) in flux-charge domain is derived from the 5th-order model (5OM) in volt-ampere domain by using the flux-charge analysis method (FCAM). The consistence of symmetry and multistability before and after dimensionality decreasing is meticulously investigated via bifurcation diagram, Lyapunov exponents, and especially attraction basins. The comparative analysis validates the effectiveness of reduction model and improves the controllability of the circuit. To avoid the noise in the analog circuit, a field-programmable gate array (FPGA) is utilized to realize the reduction model, which is rarely reported and valuable for relevant research and application.

Adaptive synchronization of a class of fractional-order complex-valued chaotic neural network with time-delay

Mei Li(李梅), Ruo-Xun Zhang(张若洵), and Shi-Ping Yang(杨世平)

Chin. Phys. B, 2021, 30 (12): 120503. DOI: 10.1088/1674-1056/abfa09

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This paper is concerned with the adaptive synchronization of fractional-order complex-valued chaotic neural networks (FOCVCNNs) with time-delay. The chaotic behaviors of a class of fractional-order complex-valued neural network are investigated. Meanwhile, based on the complex-valued inequalities of fractional-order derivatives and the stability theory of fractional-order complex-valued systems, a new adaptive controller and new complex-valued update laws are proposed to construct a synchronization control model for fractional-order complex-valued chaotic neural networks. Finally, the numerical simulation results are presented to illustrate the effectiveness of the developed synchronization scheme.

Dynamical analysis, circuit realization, and application in pseudorandom number generators of a fractional-order laser chaotic system

Chenguang Ma(马晨光), Santo Banerjee, Li Xiong(熊丽), Tianming Liu(刘天明), Xintong Han(韩昕彤), and Jun Mou(牟俊)

Chin. Phys. B, 2021, 30 (12): 120504. DOI: 10.1088/1674-1056/abfbd4

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A new five-dimensional fractional-order laser chaotic system (FOLCS) is constructed by incorporating complex variables and fractional calculus into a Lorentz-Haken-type laser system. Dynamical behavior of the system, circuit realization and application in pseudorandom number generators are studied. Many types of multi-stable states are discovered in the system. Interestingly, there are two types of state transition phenomena in the system, one is the chaotic state degenerates to a periodical state, and the other is the intermittent chaotic oscillation. In addition, the complexity of the system when two parameters change simultaneously is measured by the spectral entropy algorithm. Moreover, a digital circuit is design and the chaotic oscillation behaviors of the system are verified on this circuit. Finally, a pseudo-random sequence generator is designed using the FOLCS, and the statistical characteristics of the generated pseudo-random sequence are tested with the NIST-800-22. This study enriches the research on the dynamics and applications of FOLCS.

Explosive synchronization of multi-layer complex networks based on inter-layer star network connection

Yan-Liang Jin(金彦亮), Run-Zhu Guo(郭润珠), Xiao-Qi Yu(于晓琪), and Li-Quan Shen(沈礼权)

Chin. Phys. B, 2021, 30 (12): 120505. DOI: 10.1088/1674-1056/abff41

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Explosive synchronization (ES) is a first-order transition phenomenon that is ubiquitous in various physical and biological systems. In recent years, researchers have focused on explosive synchronization in a single-layer network, but few in multi-layer networks. This paper proposes a frequency-weighted Kuramoto model in multi-layer complex networks with star connection between layers and analyzes the factors affecting the backward critical coupling strength by both theoretical analysis and numerical validation. Our results show that the backward critical coupling strength of each layer network is influenced by the inter-layer interaction strength and the average degree. The number of network layers, the number of nodes, and the network topology can not directly affect the synchronization of the network. Enhancing the inter-layer interaction strength can prevent the emergence of explosive synchronization and increasing the average degree can promote the generation of explosive synchronization.

Stabilization strategy of a car-following model with multiple time delays of the drivers Suggested by editors

Weilin Ren(任卫林), Rongjun Cheng(程荣军), and Hongxia Ge(葛红霞)

Chin. Phys. B, 2021, 30 (12): 120506. DOI: 10.1088/1674-1056/abfcc9

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An extended car-following model with multiple delays is constructed to describe driver's driving behavior. Through stability analysis, the stability condition of this uncontrolled model is given. To dampen the negative impact of the driver's multiple delays (i.e., stability condition is not satisfied), a novel control strategy is proposed to assist the driver in adjusting vehicle operation. The control strategy consists of two parts:the design of control term as well as the design of the parameters in the term. Bifurcation analysis is performed to illustrate the necessity of the design of parameters in control terms. In the course of the design of parameters in the control term, we improve the definite integral stability method to reduce the iterations by incorporating the characteristics of bifurcation, which can determine the appropriate parameters in the control terms more quickly. Finally, in the case study, we validate the control strategy by utilizing measured data and configuring scenario, which is closer to the actual traffic. The results of validation show that the control strategy can effectively stabilize the unstable traffic flow caused by driver's delays.

Distributed optimization for discrete-time multiagent systems with nonconvex control input constraints and switching topologies

Xiao-Yu Shen(沈小宇), Shuai Su(宿帅), and Hai-Liang Hou(侯海良)

Chin. Phys. B, 2021, 30 (12): 120507. DOI: 10.1088/1674-1056/abfb5b

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This paper addresses the distributed optimization problem of discrete-time multiagent systems with nonconvex control input constraints and switching topologies. We introduce a novel distributed optimization algorithm with a switching mechanism to guarantee that all agents eventually converge to an optimal solution point, while their control inputs are constrained in their own nonconvex region. It is worth noting that the mechanism is performed to tackle the coexistence of the nonconvex constraint operator and the optimization gradient term. Based on the dynamic transformation technique, the original nonlinear dynamic system is transformed into an equivalent one with a nonlinear error term. By utilizing the nonnegative matrix theory, it is shown that the optimization problem can be solved when the union of switching communication graphs is jointly strongly connected. Finally, a numerical simulation example is used to demonstrate the acquired theoretical results.

Transition to chaos in lid-driven square cavity flow

Tao Wang(王涛) and Tiegang Liu(刘铁钢)

Chin. Phys. B, 2021, 30 (12): 120508. DOI: 10.1088/1674-1056/ac3226

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To date, there are very few studies on the transition beyond second Hopf bifurcation in a lid-driven square cavity, due to the difficulties in theoretical analysis and numerical simulations. In this paper, we study the characteristics of the third Hopf bifurcation in a driven square cavity by applying a consistent fourth-order compact finite difference scheme rectently developed by us. We numerically identify the critical Reynolds number of the third Hopf bifurcation located in the interval of (13944.7021,13946.5333) by the method of bisection. Through Fourier analysis, it is discovered that the flow becomes chaotic with a characteristic of period-doubling bifurcation when the Reynolds number is beyond the third bifurcation critical interval. Nonlinear time series analysis further ascertains the flow chaotic behaviors via the phase diagram, Kolmogorov entropy and maximal Lyapunov exponent. The phase diagram changes interestingly from a closed curve with self-intersection to an unclosed curve and the attractor eventually becomes strange when the flow becomes chaotic.

Stability analysis of hydro-turbine governing system based on machine learning

Yuansheng Chen(陈元盛) and Fei Tong(仝飞)

Chin. Phys. B, 2021, 30 (12): 120509. DOI: 10.1088/1674-1056/ac04a9

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Hydro-turbine governing system is a time-varying complex system with strong non-linearity, and its dynamic characteristics are jointly affected by hydraulic, mechanical, electrical, and other factors. Aiming at the stability of the hydro-turbine governing system, this paper first builds a dynamic model of the hydro-turbine governing system through mechanism modeling, and introduces the transfer coefficient characteristics under different load conditions to obtain the stability category of the system. BP neural network is used to perform the machine study and the predictive analysis of the stability of the system under different working conditions is carried out by using the additional momentum method to optimize the algorithm. The test set results show that the method can accurately distinguish the stability category of the hydro-turbine governing system (HTGS), and the research results can provide a theoretical reference for the operation and management of smart hydropower stations in the future.

Enhance sensitivity to illumination and synchronization in light-dependent neurons

Ying Xie(谢盈), Zhao Yao(姚昭), Xikui Hu(胡锡奎), and Jun Ma(马军)

Chin. Phys. B, 2021, 30 (12): 120510. DOI: 10.1088/1674-1056/ac1fdc

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When a phototube is activated to connect a neural circuit, the output voltage becomes sensitive to external illumination because the photocurrent across the phototube can be controlled by external electromagnetic wave. The channel currents from different branch circuits have different impacts on the outputs voltage of the neural circuit. In this paper, a phototube is incorporated into different branch circuits in a simple neural circuit, and then a light-controlled neuron is obtained for further nonlinear analysis. Indeed, the phototube is considered as exciting source when it is activated by external illumination, and two kinds of light-sensitive neurons are obtained when the phototube is connected to capacitor or induction coil, respectively. Electric synapse coupling is applied to detect possible synchronization between two functional neurons, and the energy consumption along the coupling channel via resistor is estimated. The analog circuits for the two kinds of light-sensitive neurons are supplied for further confirmation by using Multisim. It is found that two light-sensitive neurons and neural circuits can be synchronized by taming the coupling intensity carefully. It provides possible clues to understand the synchronization mechanism for eyes and artificial sensors which are sensitive to illumination. Finally, a section for open problems is supplied for further investigation about its collective behaviors in the network with/without synapse coupling.

Embedding any desired number of coexisting attractors in memristive system

Chunbiao Li(李春彪), Ran Wang(王然), Xu Ma(马旭), Yicheng Jiang(姜易成), and Zuohua Liu(刘作华)

Chin. Phys. B, 2021, 30 (12): 120511. DOI: 10.1088/1674-1056/ac1e1f

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A simple variable-boostable system is selected as the structure for hosting an arbitrarily defined memristor for chaos producing. The derived three-dimensional (3-D) memristive chaotic system shows its distinct property of offset, amplitude and frequency control. Owing its merits any desired number of coexisting attractors are embedded by means of attractor doubling and self-reproducing based on function-oriented offset boosting. In this circumstance two classes of control gates are found:one determines the number of coexisting attractors resorting to the independent offset controller while the other is the initial condition selecting any one of them. Circuit simulation gives a consistent output with theoretically predicted embedded attractors.

Controlling chaos and supressing chimeras in a fractional-order discrete phase-locked loop using impulse control

Karthikeyan Rajagopal, Anitha Karthikeyan, and Balamurali Ramakrishnan

Chin. Phys. B, 2021, 30 (12): 120512. DOI: 10.1088/1674-1056/ac1b83

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A fractional-order difference equation model of a third-order discrete phase-locked loop (FODPLL) is discussed and the dynamical behavior of the model is demonstrated using bifurcation plots and a basin of attraction. We show a narrow region of loop gain where the FODPLL exhibits quasi-periodic oscillations, which were not identified in the integer-order model. We propose a simple impulse control algorithm to suppress chaos and discuss the effect of the control step. A network of FODPLL oscillators is constructed and investigated for synchronization behavior. We show the existence of chimera states while transiting from an asynchronous to a synchronous state. The same impulse control method is applied to a lattice array of FODPLL, and the chimera states are then synchronized using the impulse control algorithm. We show that the lower control steps can achieve better control over the higher control steps.

Sensitivity to external optical feedback of circular-side hexagonal resonator microcavity laser

Tong Zhao(赵彤), Zhi-Ru Shen(申志儒), Wen-Li Xie(谢文丽), Yan-Qiang Guo(郭龑强), An-Bang Wang(王安帮), and Yun-Cai Wang(王云才)

Chin. Phys. B, 2021, 30 (12): 120513. DOI: 10.1088/1674-1056/ac322a

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The sensitivity to fault reflection is very important for larger dynamic range in fiber fault detection technique. Using time delay signature (TDS) of chaotic laser formed by optical feedback can solve the sensitivity limitation of photodetector in fiber fault detection. The TDS corresponds to the feedback position and the fault reflection can be detected by the laser diode. The sensitivity to feedback level of circular-side hexagonal resonator (CSHR) microcavity laser is numerically simulated and the feedback level boundaries of each output dynamic state are demonstrated. The peak level of TDS is utilized to analyze the sensitivity. The demonstration is presented in two aspects:the minimum feedback level when the TDS emerges and the variation degree of TDS level on feedback level changing. The results show that the CSHR microcavity laser can respond to the feedback level of 0.07%, corresponding to -63-dB feedback strength. Compared to conventional distributed feedback laser, the sensitivity improves almost 20 dB due to the shorter internal cavity length of CSHR microcavity laser. Moreover, 1% feedback level changing will induce 1.001 variation on TDS level, and this variation degree can be influenced by other critical internal parameters (active region side length, damping rate, and linewidth enhancement factor).

Cascade discrete memristive maps for enhancing chaos

Fang Yuan(袁方), Cheng-Jun Bai(柏承君), and Yu-Xia Li(李玉霞)

Chin. Phys. B, 2021, 30 (12): 120514. DOI: 10.1088/1674-1056/ac20c7

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Continuous-time memristor (CM) has been widely used to generate chaotic oscillations. However, discrete memristor (DM) has not been received adequate attention. Motivated by the cascade structure in electronic circuits, this paper introduces a method to cascade discrete memristive maps for generating chaos and hyperchaos. For a discrete-memristor seed map, it can be self-cascaded many times to get more parameters and complex structures, but with larger chaotic areas and Lyapunov exponents. Comparisons of dynamic characteristics between the seed map and cascading maps are explored. Meanwhile, numerical simulation results are verified by the hardware implementation.

Transient transition behaviors of fractional-order simplest chaotic circuit with bi-stable locally-active memristor and its ARM-based implementation

Zong-Li Yang(杨宗立), Dong Liang(梁栋), Da-Wei Ding(丁大为), Yong-Bing Hu(胡永兵), and Hao Li(李浩)

Chin. Phys. B, 2021, 30 (12): 120515. DOI: 10.1088/1674-1056/ac1fdf

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This paper proposes a fractional-order simplest chaotic system using a bi-stable locally-active memristor. The characteristics of the memristor and transient transition behaviors of the proposed system are analyzed, and this circuit is implemented digitally using ARM-based MCU. Firstly, the mathematical model of the memristor is designed, which is nonvolatile, locally-active and bi-stable. Secondly, the asymptotical stability of the fractional-order memristive chaotic system is investigated and some sufficient conditions of the stability are obtained. Thirdly, complex dynamics of the novel system are analyzed using phase diagram, Lyapunov exponential spectrum, bifurcation diagram, basin of attractor, and coexisting bifurcation, coexisting attractors are observed. All of these results indicate that this simple system contains the abundant dynamic characteristics. Moreover, transient transition behaviors of the system are analyzed, and it is found that the behaviors of transient chaotic and transient period transition alternately occur. Finally, the hardware implementation of the fractional-order bi-stable locally-active memristive chaotic system using ARM-based STM32F750 is carried out to verify the numerical simulation results.

Modeling and dynamics of double Hindmarsh-Rose neuron with memristor-based magnetic coupling and time delay

Guoyuan Qi(齐国元) and Zimou Wang(王子谋)

Chin. Phys. B, 2021, 30 (12): 120516. DOI: 10.1088/1674-1056/ac16cc

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The firing of a neuron model is mainly affected by the following factors:the magnetic field, external forcing current, time delay, etc. In this paper, a new time-delayed electromagnetic field coupled dual Hindmarsh-Rose neuron network model is constructed. A magnetically controlled threshold memristor is improved to represent the self-connected and the coupled magnetic fields triggered by the dynamic change of neuronal membrane potential for the adjacent neurons. Numerical simulation confirms that the coupled magnetic field can activate resting neurons to generate rich firing patterns, such as spiking firings, bursting firings, and chaotic firings, and enable neurons to generate larger firing amplitudes. The study also found that the strength of magnetic coupling in the neural network also affects the number of peaks in the discharge of bursting firing. Based on the existing medical treatment background of mental illness, the effects of time lag in the coupling process against neuron firing are studied. The results confirm that the neurons can respond well to external stimuli and coupled magnetic field with appropriate time delay, and keep periodic firing under a wide range of external forcing current.

Modeling the dynamics of firms' technological impact

Shuqi Xu(徐舒琪), Manuel Sebastian Mariani, and Linyuan Lü(吕琳媛)

Chin. Phys. B, 2021, 30 (12): 120517. DOI: 10.1088/1674-1056/ac364c

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Recent studies in complexity science have uncovered temporal regularities in the dynamics of impact along scientific and other creative careers, but they did not extend the obtained insights to firms. In this paper, we show that firms' technological impact patterns cannot be captured by the state-of-the-art dynamical models for the evolution of scientists' research impact, such as the Q model. Therefore, we propose a time-varying returns model which integrates the empirically-observed relation between patent order and technological impact into the Q model. The proposed model can reproduce the timing pattern of firms' highest-impact patents accurately. Our results shed light on modeling the differences behind the impact dynamics of researchers and firms.

Pitman-Yor process mixture model for community structure exploration considering latent interaction patterns

Jing Wang(王晶) and Kan Li(李侃)

Chin. Phys. B, 2021, 30 (12): 120518. DOI: 10.1088/1674-1056/ac00a1

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The statistical model for community detection is a promising research area in network analysis. Most existing statistical models of community detection are designed for networks with a known type of community structure, but in many practical situations, the types of community structures are unknown. To cope with unknown community structures, diverse types should be considered in one model. We propose a model that incorporates the latent interaction pattern, which is regarded as the basis of constructions of diverse community structures by us. The interaction pattern can parameterize various types of community structures in one model. A collapsed Gibbs sampling inference is proposed to estimate the community assignments and other hyper-parameters. With the Pitman-Yor process as a prior, our model can automatically detect the numbers and sizes of communities without a known type of community structure beforehand. Via Bayesian inference, our model can detect some hidden interaction patterns that offer extra information for network analysis. Experiments on networks with diverse community structures demonstrate that our model outperforms four state-of-the-art models.

Multilevel atomic Ramsey interferometry for precise parameter estimations

X N Feng(冯夏宁) and L F Wei(韦联福)

Chin. Phys. B, 2021, 30 (12): 120601. DOI: 10.1088/1674-1056/ac0527

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Multi-path (or multi-mode) entanglement has been proved to be a useful resource for sub-shot-noise sensitivity of phase estimation, which has aroused much research interest in quantum metrology recently. Various schemes of multi-path interferometers based on optical systems have been put forward. Here, we study a multi-state interferometer with multi-level atoms by projective measurements. Specifically, we investigate its ultimate sensitivity described by quantum Fisher information theory and find that the Cramer-Rao bound can be achieved. In particular, we investigate a specific scheme to improve the sensitivity of magnetometery with a three-state interferometry delivered by a single nitrogen-vacancy (NV) center of diamond with tailor pulses. The impacts of imperfections of the atomic beam-splitter, described by the three-level quantum Fourier transform, on the sensitivity of phase estimation is also discussed.

A terahertz on-chip InP-based power combiner designed using coupled-grounded coplanar waveguide lines

Huali Zhu(朱华利), Yong Zhang(张勇), Kun Qu(屈坤), Haomiao Wei(魏浩淼), Yukun Li(黎雨坤), Yuehang Xu(徐跃杭), and Ruimin Xu(徐锐敏)

Chin. Phys. B, 2021, 30 (12): 120701. DOI: 10.1088/1674-1056/ac05af

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This article presents the design and performance of a terahertz on-chip coupled-grounded coplanar waveguide (GCPW) power combiner using a 50 μm-thick InP process. The proposed topology uses two coupled-GCPW lines at the end of the input port to substitute two quarter-wavelength GCPW lines, which is different from the conventional Wilkinson power combiner and can availably minimize the coverage area. According to the results obtained, for the frequency range of 210-250 GHz, the insertion losses for each two-way combiner and four-way combiner were lower than 1.05 dB and 1.35 dB, respectively, and the in-band return losses were better than 11 dB. Moreover, the proposed on-chip GCPW-based combiners achieved a compromise in low-loss, broadband, and small-size, which can find wide applications in terahertz bands, such as power amplifiers and signal distribution networks.

Design of a novel correlative reflection electron microscope for in-situ real-time chemical analysis

Tian-Long Li(李天龙), Zheng Wei(魏征), and Wei-Shi Wan(万唯实)

Chin. Phys. B, 2021, 30 (12): 120702. DOI: 10.1088/1674-1056/ac0522

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A novel instrument that integrates reflection high energy electron diffraction (RHEED), electron energy loss spectroscopy (EELS), and imaging is designed and simulated. Since it can correlate the structural, elemental, and spatial information of the same surface region via the simultaneously acquired patterns of RHEED, EELS, and energy-filtered electron microscopy, it is named correlative reflection electron microscopy (c-REM). Our simulation demonstrates that the spatial resolution of this c-REM is lower than 50 nm, which meets the requirements for in-situ monitoring the structural and chemical evolution of surface in advanced material.

A 32-channel 100 GHz wavelength division multiplexer by interleaving two silicon arrayed waveguide gratings Suggested by editors

Changjian Xie(解长健), Xihua Zou (邹喜华), Fang Zou(邹放), Lianshan Yan(闫连山), Wei Pan(潘炜), and Yong Zhang(张永)

Chin. Phys. B, 2021, 30 (12): 120703. DOI: 10.1088/1674-1056/ac2d23

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A 32-channel wavelength division multiplexer with 100 GHz spacing is designed and fabricated by interleaving two silicon arrayed waveguide gratings (AWGs). It has a parallel structure consisting of two silicon 16-channel AWGs with 200 GHz spacing and a Mach-Zehnder interferometer (MZI) with 200 GHz free spectral range. The 16 channels of one silicon AWG are interleaved with those of the other AWG in spectrum, but with an identical spacing of 200 GHz. For the composed wavelength division multiplexer, the experiment results reveal 32 wavelength channels in C-band, a wavelength spacing of 100 GHz, and a channel crosstalk lower than -15 dB.

Photoluminescence of green nanophosphors Sr₂MgSi₂O₇ doped with Tb³⁺ under 374-nm excitation

Bo-Shi Mu(牟博石), Yi Zhang(张熠), Qing-Feng Bian(边庆丰), Cheng-Ren Li(李成仁), Zhi-Chao Li(李志超), Yun-Ting Chu(褚云婷), Feng Zhao(赵峰), and Jing-Chang Sun(孙景昌)

Chin. Phys. B, 2021, 30 (12): 123201. DOI: 10.1088/1674-1056/abfbce

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A series of Sr₂MgSi₂O₇:Tb³⁺ nanophosphors is prepared using a high-temperature solid-state reaction. The x-ray diffraction patterns show that the crystal structure of the sample is not significantly affected by Tb³⁺ ions. However, the images of the scanning electron microscope illustrate that the average size of nanoparticles becomes larger with the increase of Tb³⁺ concentration. Unlike earlier investigations on down-conversion emission of Tb³⁺ ion excited by deep ultraviolet light, in this work, the photoluminescence characteristics of Sr₂MgSi₂O₇ nanophosphors doped with different Tb³⁺ concentrations are analyzed under 374-nm excitations. The intense green emission at 545 nm is observed at an optimal doping concentration of 1.6 mol%. The main reason for the concentration quenching is due to the electric dipole-electric dipole interaction among Tb³⁺ ions.

Taking snapshots of a moving electron wave packet in molecules using photoelectron holography in strong-field tunneling ionization

Mingrui He(何明睿), Yang Fan(樊洋), Yueming Zhou(周月明), and Peixiang Lu(陆培祥)

Chin. Phys. B, 2021, 30 (12): 123202. DOI: 10.1088/1674-1056/abf7a7

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Coherent superposition of electronic states induces attosecond electron motion in molecules. We theoretically investigate the strong-field ionization of this superposition state by numerically solving the time-dependent Schrödinger equation. In the obtained photoelectron momentum distribution, an intriguing bifurcation structure appears in the strong-field holographic interference pattern. We demonstrate that this bifurcation structure directly provides complete information about the status of the transient wave function of the superposition state:the horizontal location of the bifurcation in the momentum distribution reveals the relative phase of the involved components of the superposition state and the vertical position indicates the relative coefficient. Thus, this bifurcation structure takes a snapshot of the transient electron wave packet of the superposition state and provides an intuitive way to monitor electron motion in molecules.

Exploration of magnetic field generation of H₃²⁺ by direc ionization and coherent resonant excitation Suggested by editors

Zhi-Jie Yang(杨志杰), Qing-Yun Xu(徐清芸), Yong-Lin He(何永林), Xue-Shen Liu(刘学深), and Jing Guo(郭静)

Chin. Phys. B, 2021, 30 (12): 123203. DOI: 10.1088/1674-1056/ac0346

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Coherent electronic dynamics are of great significance in photo-induced processes and molecular magnetism. We theoretically investigate electronic dynamics of triatomic molecule H₃²⁺ by circularly polarized pulses, including electron density distributions, induced electronic currents, and ultrafast magnetic field generation. By comparing the results of the coherent resonant excitation and direct ionization, we found that for the coherent resonant excitation, the electron is localized and the coherent electron wave packet moves periodically between three protons, which can be attributed to the coherent superposition of the ground A' state and excited E⁺ state. Whereas, for the direct single-photon ionization, the induced electronic currents mainly come from the free electron in the continuum state. It is found that there are differences in the intensity, phase, and frequency of the induced current and the generated magnetic field. The scheme allows one to control the induced electronic current and the ultrafast magnetic field generation.

Theoretical calculations of hyperfine splitting, Zeeman shifts, and isotope shifts of ²⁷Al⁺ and logical ions in Al⁺ clocks Suggested by editors

Xiao-Kang Tang(唐骁康), Xiang Zhang(张祥), Yong Shen(沈咏), and Hong-Xin Zou(邹宏新)

Chin. Phys. B, 2021, 30 (12): 123204. DOI: 10.1088/1674-1056/ac0130

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Based on the multiconfiguration Dirac-Hartree-Fock (MCDHF) method, similar models are employed to simultaneously calculate the first-order and second-order Zeeman coefficients as well as the hyperfine interaction constants of the related energy levels of ²⁷Al⁺ and its logical ions ⁹Be⁺ and ²⁵Mg⁺ in the ²⁷Al⁺ optical clock. With less than 0.34% deviations from experimental values in Zeeman coefficients of ²⁷Al⁺, these calculated parameters will be of great help for better evaluation of the systematic uncertainty. We also calculate the isotope shift parameters of the related energy levels, which could extend our knowledge and understanding of nuclear properties of these ions.

Line positions, intensities, and Einstein A coefficients for 3-0 band of ¹²C¹⁶O: A spectroscopy learning method

Zhi-Xiang Fan(范志祥), Zhi-Zhang Ni(倪志樟), Jie-Jie He(贺洁洁), Yi-Fan Wang(王一凡), Qun-Chao Fan(樊群超), Jia Fu(付佳), Yong-Gen Xu(徐勇根), Hui-Dong Li(李会东), Jie Ma(马杰), and Feng Xie(谢锋)

Chin. Phys. B, 2021, 30 (12): 123301. DOI: 10.1088/1674-1056/ac3069

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Based on the model- and data-driven strategy, a spectroscopy learning method that can extract the novel and hidden information from the line list databases has been applied to the R branch emission spectra of 3-0 band of the ground electronic state of ¹²C¹⁶O. The labeled line lists such as line intensities and Einstein A coefficients quoted in HITRAN2020 are collected to enhance the dataset. The quantified spectroscopy-learned spectroscopic constants is beneficial for improving the extrapolative accuracy beyond the measurements. Explicit comparisons are made for line positions, line intensities, Einstein A coefficients, which demonstrate that the model- and data-driven spectroscopy learning approach is a promising and an easy-to-implement strategy.

Theoretical verification of intermolecular hydrogen bond induced thermally activated delayed fluorescence in SOBF-Ome

Mu-Zhen Li(李慕臻), Fei-Yan Li(李飞雁), Qun Zhang(张群), Kai Zhang(张凯), Yu-Zhi Song(宋玉志), Jian-Zhong Fan(范建忠), Chuan-Kui Wang(王传奎), and Li-Li Lin(蔺丽丽)

Chin. Phys. B, 2021, 30 (12): 123302. DOI: 10.1088/1674-1056/ac1b91

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Thermally activated delayed fluorescence (TADF) molecules have attracted great attention as high efficient luminescent materials. Most of TADF molecules possess small energy gap between the first singlet excited state (S₁) and the first triplet excited state (T₁) to favor the up-conversion from T₁ to S₁. In this paper, a new TADF generation mechanism is revealed based on theoretical simulation. By systematic study of the light-emitting properties of SOBF-OMe in both toluene and in aggregation state, we find that the single SOBF-OMe could not realize TADF emission due to large energy gap as well as small up-conversion rates between S₁ and T₁. Through analysis of dimers, we find that dimers with intermolecular hydrogen bond (H-bond) are responsible for the generation of TADF, since smaller energy gap between S₁ and T₁ is found and the emission wavelength is in good agreement with experimental counterpart. The emission properties of SOBF-H are also studied for comparison, which reflect the important role of H-bond. Our theoretical results agree ith experimental results well and confirm the mechanism of H-bond induced TADF.

Elastic electron scattering with formamide-(H₂O)_n complexes (n=1, 2): Influence of microsolvation on the π^* and σ^* resonances

Kedong Wang(王克栋), Yan Wang(王言), Jie Liu(刘洁), Yiwen Wang(王怡文), and Haoxing Zhang(张浩兴)

Chin. Phys. B, 2021, 30 (12): 123401. DOI: 10.1088/1674-1056/abf7a8

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We report elastic cross sections for low-energy electron scattering with formamide-(H₂O)_n complexes (n=1, 2) in the energy region of 0.01-8 eV. The scattering calculations are performed using the R-matrix method in the static-exchange (SE) approximation. We consider three structures of formamide-H₂O and six structures of formamide-(H₂O)₂ in the present work. Our purpose is to investigate effects of water molecules hydrogen-bonding to formamide. We focus on the influence of microsolvation on the π^* and σ^* resonances of formamide. The scattering result for complexes shows that the position of π^* resonance appears at lower or higher energies in the cluster than in the isolated formamide depending on the complex structure and the water role in the hydrogen bonding. We explain this behavior according to the net charge of the solute. It is found that the microsolvation environment has a substantial effect on the width of π^* resonance. Our results indicate that surrounding water molecules may affect the lifetime of the resonances, and hence the process is driven by the anion state, such as the dissociative electron attachment.

Multiple recollisions in nonsequential double ionization below the recollision-ionization threshold

Xiao-Meng Ma(马晓萌), Ai-Hong Tong(童爱红), Zhuo Wang(王茁), and Chun-Yang Zhai(翟春洋)

Chin. Phys. B, 2021, 30 (12): 123402. DOI: 10.1088/1674-1056/ac2d1e

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By using the three-dimensional classical ensemble model, the recollision dynamics in nonsequential double ionization (NSDI) of Ar by 780-nm laser pulses at (6-1.2)×10¹⁴ W/cm² was extensively studied. We revealed the picture of multiple-recollision in the double ionization events at the laser intensity region below the recollision-ionization threshold. Via tracing the NSDI trajectories, it was found that the contribution of these multiple-recollision events increases as the laser intensity decreases. In this low intensity region, many multiple-recollision induced NSDI trajectories occur through the doubly excited states. The decay speed of the doubly excited state decreases with the decreasing laser intensity.

Exact quantum dynamics study of the H(²S)+SiH⁺(X¹Σ⁺) reaction on a new potential energy surface of SiH₂⁺(X²A₁)

Wen-Li Zhao(赵文丽), Rui-Shan Tan(谭瑞山), Xue-Cheng Cao(曹学成), Feng Gao(高峰), and Qing-Tian Meng(孟庆田)

Chin. Phys. B, 2021, 30 (12): 123403. DOI: 10.1088/1674-1056/ac1b8c

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Based on a new global potential energy surface of SiH₂⁺(X²A₁), the exact quantum dynamical calculation for the H(²S)+SiH⁺(X¹Σ⁺)→H₂+Si⁺ reaction has been carried out by using the Chebyshev wave packet method. The initial state specified (ν_i=0, j_i=0) probabilities, integral cross sections (ICS) and thermal rate constants of the title reaction are calculated. All partial wave contributions up to J=90 are calculated in exact quantum calculation including the full Coriolis coupling (CC) effect. The dynamical behaviors of probabilities, ICSs and rate constants are found to be in accord with an exothermic reaction without potential barrier. By comparing the probabilities of CC with the corresponding centrifugal sudden (CS) approximation ones, it can be concluded that neglecting CC effect will decrease the collision time, increase the amplitude of oscillation and lead to overestimation or underestimation of the reaction probability. For ICSs and rate constants, it is found that the deviation of CC and CS ICSs is small in the most of collision energy range except for the range of 0 eV-0.05 eV, while the deviation of both rate constants is considerable in the temperature range of 16 K-1000 K.

Enhancing stationary entanglement between two optomechanical oscillators by Coulomb interaction with Kerr medium

Tian-Le Yang(杨天乐), Chen-Long Zhu(朱陈龙), Sheng Liu(刘声), and Ye-Jun Xu(许业军)

Chin. Phys. B, 2021, 30 (12): 124201. DOI: 10.1088/1674-1056/abff3e

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We theoretically study the stationary entanglement of two charged nanomechanical oscillators coupling via Coulomb interaction in an optomechanical system with an additional Kerr medium. We show that the degree of entanglement between two nanomechanical oscillators is suppressed by Kerr interaction due to photon blockade and enhanced by Coulomb coupling strength. We also show other parameters for adjusting and obtaining entanglement, such as the driving power and the frequencies of the two oscillators, and the entanglement is robust against temperature. Our study proves a way for adjusting stationary entanglement between two optomechanical oscillators by Coulomb interaction and Kerr medium.

Broad gain, continuous-wave operation of InP-based quantum cascade laser at λ~11.8 μm Suggested by editors

Huan Wang(王欢), Jin-Chuan Zhang(张锦川), Feng-Min Cheng(程凤敏), Zeng-Hui Gu(顾增辉), Ning Zhuo(卓宁), Shen-Qiang Zhai(翟慎强), Feng-Qi Liu(刘峰奇), Jun-Qi Liu(刘俊岐), Shu-Man Liu(刘舒曼), and Zhan-Guo Wang(王占国)

Chin. Phys. B, 2021, 30 (12): 124202. DOI: 10.1088/1674-1056/abf91a

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We demonstrate a broad gain, continuous-wave (CW) operation InP-based quantum cascade laser (QCL) emitting at 11.8 μm with a modified dual-upper-state (DAU) and diagonal transition active region design. A 3 mm cavity length, 16.5 μm average ridge wide QCL with high-reflection (HR) coatings demonstrates a maximum peak power of 1.07 W at 283 K and CW output power of 60 mW at 293 K. The device also shows a broad and dual-frequency lasing spectrum in pulsed mode and a maximum average power of 258.6 mW at 283 K. Moreover, the full width at half maximum (FWHM) of the electroluminescent spectrum measured at subthreshold current is 2.37 μm, which indicates a broad gain spectrum of the materials. The tuning range of 1.38 μm is obtained by a grating-coupled external cavity (EC) Littrow configuration, which is beneficial for gas detection.

Brillouin gain spectrum characterization in Ge-doped large-mode-area fibers

Xia-Xia Niu(牛夏夏), Yi-Feng Yang(杨依枫), Zhao Quan(全昭), Chun-Lei Yu(于春雷), Qin-Ling Zhou(周秦岭), Hui Shen(沈辉), Bing He(何兵), and Jun Zhou(周军)

Chin. Phys. B, 2021, 30 (12): 124203. DOI: 10.1088/1674-1056/abf7ab

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The dependence of Brillouin gain spectrum (BGS) characteristics, including the Brillouin frequency shift (BFS) and the BGS bandwidth, on germanium concentration in large-mode-area Ge-doped passive fibers is investigated theoretically and experimentally. The simulation results show that the BFS is inversely proportional to GeO₂ concentration, and the BGS bandwidth initially increases with the augment of GeO₂ concentration, and then decreases. The BGSs of four fibers with core diameters of 10 μm and 20 μm for different GeO₂ concentrations are compared experimentally. Experimental results demonstrate that with the same core diameter, the variations of BFS and BGS bandwidths with GeO₂ concentration accord with the simulation results. Additionally, the BGS characteristics of three large-mode-area passive fibers with diameters of 10 μm, 25 μm, and 30 μm are measured, which confirm that the increasing of the fiber diameters will cause the BGS bandwidth to broaden. We believe that these results can provide valuable references for modulating the high-power narrow-linewidth fiber lasers and Brillouin fiber amplifiers.

Optical solitons supported by finite waveguide lattices with diffusive nonlocal nonlinearity

Changming Huang(黄长明), Hanying Deng(邓寒英), Liangwei Dong(董亮伟), Ce Shang(尚策), Bo Zhao(赵波), Qiangbo Suo(索强波), and Xiaofang Zhou(周小芳)

Chin. Phys. B, 2021, 30 (12): 124204. DOI: 10.1088/1674-1056/abf555

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We investigate the properties of fundamental, multi-peak, and multi-peaked twisted solitons in three types of finite waveguide lattices imprinted in photorefractive media with asymmetrical diffusion nonlinearity. Two opposite soliton self-bending signals are considered for different families of solitons. Power thresholdless fundamental and multi-peaked solitons are stable in the low power region. The existence domain of two-peaked twisted solitons can be changed by the soliton self-bending signals. When solitons tend to self-bend toward the waveguide lattice, stable two-peaked twisted solitons can be found in a larger region in the middle of their existence region. Three-peaked twisted solitons are stable in the lower (upper) cutoff region for a shallow (deep) lattice depth. Our results provide an effective guidance for revealing the soliton characteristics supported by a finite waveguide lattice with diffusive nonlocal nonlinearity.

Single-mode antiresonant terahertz fiber based on mode coupling between core and cladding Suggested by editors

Shuai Sun(孙帅), Wei Shi(史伟), Quan Sheng(盛泉), Shijie Fu(付士杰), Zhongbao Yan(闫忠宝), Shuai Zhang(张帅), Junxiang Zhang(张钧翔), Chaodu Shi(史朝督), Guizhong Zhang(张贵忠), and Jianquan Yao(姚建铨)

Chin. Phys. B, 2021, 30 (12): 124205. DOI: 10.1088/1674-1056/ac032e

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Based on the index-induced mode coupling between the higher-order mode in core and the fundamental mode in cladding tubes, the single-mode operation can be realized in any antiresonant fibers (ARFs) when satisfying that the area ratio of cladding tube and core is about 0.46:1, and this area ratio also should be modified according to the shape and the number of cladding tubes. In the ARF with nodal core boundary, the mode in core also can couple with the mode in the wall of core boundary, which can further enhance the suppression of high-order mode. Accordingly, an ARF with conjoint semi-elliptical cladding tubes realizes a loss of higher-order mode larger than 30 dB/m; simultaneously, a loss of fundamental mode loss less than 0.4 dB/m.

A 61-mJ, 1-kHz cryogenic Yb: YAG laser amplifier

Huijun He(何会军), Jun Yu(余军), Wentao Zhu(朱文涛), Qingdian Lin(林庆典), Xiaoyang Guo(郭晓杨), Cangtao Zhou(周沧涛), and Shuangchen Ruan(阮双琛)

Chin. Phys. B, 2021, 30 (12): 124206. DOI: 10.1088/1674-1056/ac140d

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We report a diode-pumped rod-type Yb:YAG laser amplifier operating at 1 kHz. Cryogenic cooling method was adopted to make the Yb:YAG crystal work with four-level behavior. A single-frequency fiber laser acts as the seed in an actively Q-switched Yb:YAG oscillator. The resonator delivers 5.75-mJ pulses at 1 kHz with a pulse duration of approximately 40 ns. The pulses were amplified to 61 mJ in a four-pass rod-type Yb:YAG amplifier with optical-to-optical efficiency of 24% in the main amplifier. The M² parameter of the output laser is <1.4.

Ghost imaging-based optical cryptosystem for multiple images using integral property of the Fourier transform

Yi Kang(康祎), Leihong Zhang(张雷洪), Hualong Ye(叶华龙), Dawei Zhang(张大伟), and Songlin Zhuang(庄松林)

Chin. Phys. B, 2021, 30 (12): 124207. DOI: 10.1088/1674-1056/ac0815

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A novel ghost imaging-based optical cryptosystem for multiple images using the integral property of the Fourier transform is proposed. Different from other multiple-image encryption schemes, we mainly construct the modulation patterns related to the plaintext images to realize the encrypted transmission of multiple images. In encryption process, the first image is encrypted by the ghost imaging encryption scheme, and the intensity sequence obtained by the bucket detector is used as the ciphertext. Then modulation patterns of other images are constructed by using the integral property of the Fourier transform and used as the keys. Finally, the ciphertext and keys are transmitted to the receiver to complete the encryption process. During decryption, the receiver uses different keys to decrypt the ciphertext and gets different plaintext images, and decrypted images have no image aliasing problem. Experiments and simulations verify the feasibility, security, and robustness of the proposed scheme. This scheme has high scalability and broad application prospect, which provides a new idea for optical information encryption.

Asymmetric coherent rainbows induced by liquid convection Suggested by editors

Tingting Shi(施婷婷), Xuan Qian(钱轩), Tianjiao Sun(孙天娇), Li Cheng(程力), Runjiang Dou(窦润江), Liyuan Liu(刘力源), and Yang Ji(姬扬)

Chin. Phys. B, 2021, 30 (12): 124208. DOI: 10.1088/1674-1056/ac0039

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Coherent rainbows can be formed by focusing white-light laser into liquids. They are bilaterally symmetric interference rings with various shapes. Such interference rings arise from the temperature distribution of the liquid induced by laser heating, i.e., thermal lens effect, which changes the refractive index locally and thus the optical path difference. The up-down asymmetry of the interference rings is caused by convection in the liquid. With the increase of the viscosity, the interference rings change their shape from oval to circular shape. After a shutter is opened and the laser shines into the liquid, the interference rings are circular at the beginning. As time goes on, they gradually turn into an oval shape. Let the liquid go a free-fall at the beginning, the interference rings remain circular. All the three experiments have confirmed that the asymmetric interference rings are due to convection in the liquid associated with thermal lens effect. We also numerically simulate the two-dimensional heat conduction with and without convection, whose results agree well with our experimental observations.

Computational ghost imaging with deep compressed sensing Suggested by editors

Hao Zhang(张浩), Yunjie Xia(夏云杰), and Deyang Duan(段德洋)

Chin. Phys. B, 2021, 30 (12): 124209. DOI: 10.1088/1674-1056/ac0042

Abstract ( 386 )

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Computational ghost imaging (CGI) provides an elegant framework for indirect imaging, but its application has been restricted by low imaging performance. Herein, we propose a novel approach that significantly improves the imaging performance of CGI. In this scheme, we optimize the conventional CGI data processing algorithm by using a novel compressed sensing (CS) algorithm based on a deep convolution generative adversarial network (DCGAN). CS is used to process the data output by a conventional CGI device. The processed data are trained by a DCGAN to reconstruct the image. Qualitative and quantitative results show that this method significantly improves the quality of reconstructed images by jointly training a generator and the optimization process for reconstruction via meta-learning. Moreover, the background noise can be eliminated well by this method.

Refocusing and locating effect of fluorescence scattering field

Jian-Gong Cui(崔建功), Ya-Xin Yu(余亚鑫), Xiao-Xia Chu(楚晓霞), Rong-Yu Zhao(赵荣宇), Min Zhu(祝敏), Fan Meng(孟凡), and Wen-Dong Zhang(张文栋)

Chin. Phys. B, 2021, 30 (12): 124210. DOI: 10.1088/1674-1056/ac2804

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Optical imaging deep inside scattering medium has always been one of the challenges in the field of bioimaging, which significantly drawbacks the employment of con-focal microscopy system. Although a variety of feedback techniques, such as acoustic or nonlinear fluorescence-based schemes have realized the refocusing of the coherent light, the problems of non-invasively refocusing and locating of linearly-excited fluorescent beads inside the scattering medium have not been thoroughly explored. In this paper, we linearly excited the fluorescent beads inside a scattering medium by using our homemade optical con-focal system, collected the fluorescence scattering light as the optimized target, and established a theoretical model of target contrast enhancement, which is consistent with the experimental data. By improving both the cost function and variation rate within the genetic algorithm, we could refocus the fluorescence scattering field while improving the contrast enhancement factor to 12.8 dB. Then, the positions of the fluorescent beads are reconstructed by sub-pixel accuracy centroid localization algorithm, and the corresponding error is no more than 4.2 μ with several fluorescent beads within the field of view. Finally, the main factors such as the number of fluorescent beads, the thickness of the scattering medium, the modulating parameter, the experimental noise and the system long-term stability are analyzed and discussed in detail. This study proves the feasibility of reconstructing fluorescent labeled cells inside biological tissues, which provides certain reference value for deep imaging of biological tissues.

Phase behavior of rotationally asymmetric Brownian kites containing 90° internal angles Suggested by editors

Huaqing Liu(柳华清), Yiwu Zong(宗奕吾), Zhanglin Hou(侯章林), Thomas G. Mason, and Kun Zhao(赵坤)

Chin. Phys. B, 2021, 30 (12): 124701. DOI: 10.1088/1674-1056/ac306c

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Previous Monte Carlo simulations have shown that ordered tetratic phases can emerge in a dense two-dimensional Brownian system of rotationally asymmetric hard kites having 90° internal angles. However, there have been no experimental investigations yet to compare with these simulation results. Here, we have fabricated two types of micron-sized kites having internal angles of 72°-90°-108°-90° and 72°-99°-90°-99°, respectively, and we have experimentally studied their phase behavior in two-dimensional systems. Interestingly and in contrast to the Monte Carlo simulations, the experimental results show a phase sequence of isotropic fluid-hexagonal rotator crystal-square crystal as the area fraction φ_A increases for both types of kites. The observed square crystal displays not only a quasi-long-range translational order but also (quasi-)long-range 4-fold bond- and molecular-orientational order; these characteristics confirm that tetratic order can emerge even in dense Brownian systems of rotationally asymmetric particles. A model based on local polymorphic configurations (LPCs) is proposed to understand the origin of the square lattice order in these dense kite systems. The results in this study provide a new route to realize custom-designed self-assembly of colloids by controlling LPCs.

Effect of deformation of diamond anvil and sample in diamond anvil cell on the thermal conductivity measurement Suggested by editors

Caihong Jia(贾彩红), Dawei Jiang(蒋大伟), Min Cao(曹敏), Tingting Ji(冀婷婷), and Chunxiao Gao(高春晓)

Chin. Phys. B, 2021, 30 (12): 124702. DOI: 10.1088/1674-1056/ac11d1

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Studies show that the sample thickness is an important parameter in investigating the thermal transport properties of materials under high-temperature and high-pressure (HTHP) in the diamond anvil cell (DAC) device. However, it is an enormous challenge to measure the sample thickness accurately in the DAC under severe working conditions. In conventional methods, the influence of diamond anvil deformation on the measuring accuracy is ignored. For a high-temperature anvil, the mechanical state of the diamond anvil becomes complex and is different from that under the static condition. At high temperature, the deformation of anvil and sample would be aggravated. In the present study, the finite volume method is applied to simulate the heat transfer mechanism of stable heating DAC through coupling three radiative-conductive heat transfer mechanisms in a high-pressure environment. When the temperature field of the main components is known in DAC, the thermal stress field can be analyzed numerically by the finite element method. The obtained results show that the deformation of anvil will lead to the obvious radial gradient distribution of the sample thickness. If the top and bottom surfaces of the sample are approximated to be flat, it will be fatal to the study of the heat transport properties of the material. Therefore, we study the temperature distribution and thermal conductivity of the sample in the DAC by thermal-solid coupling method under high pressure and stable heating condition.

Fractal microstructure of Ag film via plasma discharge as SERS substrates

Xue-Fen Kan(阚雪芬), Cheng Yin(殷澄), Zhuang-Qi Cao(曹庄琪), Wei Su(苏巍), Ming-Lei Shan(单鸣雷), and Xian-Ping Wang(王贤平)

Chin. Phys. B, 2021, 30 (12): 125201. DOI: 10.1088/1674-1056/ac0693

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According to the atmospheric pressure plasma (APP) technology, we propose a rapid synthetic approach of the substrates for enhanced Raman spectroscopy. The plasma is used to modify and etch the surface of silver film, which generates large scale hotspots' aggregation. By switching the discharge polarity and adjusting the film thickness, different surface morphologies are formed due to the oxidation, reactive etch and accumulation of the plasma product in a certain space. Especially under positive corona discharge condition, dense snake-like microstructures are formed by the gradual connection of individual nanoparticles, which are driven by the influence of the electric field on surface diffusion. In addition, the experiments verify that the corresponding enhancement factor (EF) raises at least five orders of magnitude and the treatment time is about 10 min.

Landau damping of electrons with bouncing motion in a radio-frequency plasma

Jun Tao(陶军), Nong Xiang(项农), Yemin Hu(胡业民), and Yueheng Huang(黄跃恒)

Chin. Phys. B, 2021, 30 (12): 125202. DOI: 10.1088/1674-1056/ac16cd

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One-dimensional particle simulations have been conducted to study the interaction between a radio-frequency electrostatic wave and electrons with bouncing motion. It is shown that bounce resonance heating can occur at the first few harmonics of the bounce frequency (nω_b,n=1,2,3,...). In the parameter regimes in which bounce resonance overlaps with Landau resonance, the higher harmonic bounce resonance may accelerate electrons at the velocity much lower than the wave phase velocity to Landau resonance region, enhancing Landau damping of the wave. Meanwhile, Landau resonance can increase the number of electrons in the lower harmonic bounce resonance region. Thus electrons can be efficiently heated. The result might be applicable for collisionless electron heating in low-temperature plasma discharges.

Actively tunable dual-broadband graphene-based terahertz metamaterial absorber

Dan Hu(胡丹), Tian-Hua Meng(孟田华), Hong-Yan Wang(王红燕), and Mai-Xia Fu(付麦霞)

Chin. Phys. B, 2021, 30 (12): 126101. DOI: 10.1088/1674-1056/ac0794

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A tunable metamaterial absorber (MA) with dual-broadband and high absorption properties at terahertz (THz) frequencies is designed in this work. The MA consists of a periodic array of flower-like monolayer graphene patterns at top, a SiO₂ dielectric spacer in middle, and a gold ground plane at the bottom. The simulation results demonstrate that the designed MA has two wide absorption bands with an absorption of over 90% in frequency ranges of 0.68 THz-1.63 THz and 3.34 THz-4.08 THz, and the corresponding relative bandwidths reach 82.3% and 20%, respectively. The peak absorptivity of the absorber can be dynamically controlled from less than 10% to nearly 100% by adjusting the graphene chemical potential from 0 eV to 0.9 eV. Furthermore, the designed absorber is polarization-insensitive and has good robustness to incident angles. Such a high-performance MA has broad application prospects in THz imaging, modulating, filtering, etc.

Band alignment in SiC-based one-dimensional van der Waals homojunctions

Xing-Yi Tan(谭兴毅), Lin-Jie Ding(丁林杰), and Da-Hua Ren(任达华)

Chin. Phys. B, 2021, 30 (12): 126102. DOI: 10.1088/1674-1056/abfbd2

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The density functional theory method is utilized to verify the electronic structures of SiC nanotubes (SiCNTs) and SiC nanoribbons (SiCNRs) one-dimensional (1D) van der Waals homojunctions (vdWh) under an applied axial strain and an external electric field. According to the calculated results, the SiCNTs/SiCNRs 1D vdWhs are direct semiconductors with a type-II band alignment and robust electronic structures with different diameters or widths. Furthermore, the SiCNTs/SiCNRs 1D vdWhs are direct semiconductors with a type-I band alignment, respectively, in a range of[-0.3, -0.1] V/Å and[0.1, 0.3] V/Å and change into metal when the electric field intensity is equal to or higher than 0.4 V/Å. Interestingly, the SiCNTs/SiCNRs 1D vdWhs have robust electronic structures under axial strain. These findings demonstrate theoretically that the SiCNTs/SiCNRs 1D vdWhs can be employed in nanoelectronics devices.

Suppression of persistent photoconductivity in high gain Ga₂O₃ Schottky photodetectors Suggested by editors

Haitao Zhou(周海涛), Lujia Cong(丛璐佳), Jiangang Ma(马剑钢), Bingsheng Li(李炳生), Haiyang Xu(徐海洋), and Yichun Liu(刘益春)

Chin. Phys. B, 2021, 30 (12): 126104. DOI: 10.1088/1674-1056/ac2d1b

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The defect-related photoconductivity gain and persistent photoconductivity (PPC) observed in Ga₂O₃ Schottky photodetectors lead to a contradiction between high responsivity and fast recovery speed. In this work, a metal-semiconductor-metal (MSM) Schottky photodetector, a unidirectional Schottky photodetector, and a photoconductor were constructed on Ga₂O₃ films. The MSM Schottky devices have high gain (> 13) and high responsivity (> 2.5 A/W) at 230-250 nm, as well as slow recovery speed caused by PPC. Interestingly, applying a positive pulse voltage to the reverse-biased Ga₂O₃/Au Schottky junction can effectively suppress the PPC in the photodetector, while maintaining high gain. The mechanisms of gain and PPC do not strictly follow the interface trap trapping holes or the self-trapped holes models, which is attributed to the correlation with ionized oxygen vacancies in the Schottky junction. The positive pulse voltage modulates the width of the Schottky junction to help quickly neutralize electrons and ionized oxygen vacancies. The realization of suppression PPC functions and the establishment of physical models will facilitate the realization of high responsivity and fast response Schottky devices.

Ab initio study of dynamical properties of U-Nb alloy melt Suggested by editors

Yong-Peng Shi(时永鹏), Ming-Feng Liu(刘鸣凤), Yun Chen(陈云), Wen-Lin Mo(莫文林), Dian-Zhong Li(李殿中), Tao Fa(法涛), Bin Bai(白彬), Xiao-Lin Wang(汪小琳), and Xing-Qiu Chen(陈星秋)

Chin. Phys. B, 2021, 30 (12): 126105. DOI: 10.1088/1674-1056/ac0bac

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The U-Nb alloy, as a kind of nuclear material with good corrosion resistance and mechanical properties, plays an important role in the nuclear industry. However, the experimental measurements and theoretical calculations of many parameters which are essential in describing the dynamical properties of this alloy melt, including density, diffusivity, and viscosity, have not been carried out yet. The lack of data on the dynamical properties of nuclear materials seriously hinders the high-performance nuclear materials from being developed and applied. In this work, the dynamical properties of the U-Nb alloy melt are systematically studied by means of ab initio molecular dynamics simulations and their corresponding mathematical models are established, thereby being able to rapidly calculate the densities, diffusion coefficients, viscosities, and their activation energies in the whole U-Nb liquid region. This work provides a new idea for investigating the dynamical properties of binary alloy melts, thereby promoting the development of melt research.

Synthesis of flower-like WS₂ by chemical vapor deposition

Jin-Zi Ding(丁金姿), Wei Ren(任卫), Ai-Ling Feng(冯爱玲), Yao Wang(王垚), Hao-Sen Qiao(乔浩森), Yu-Xin Jia(贾煜欣), Shuang-Xiong Ma(马双雄), and Bo-Yu Zhang(张博宇)

Chin. Phys. B, 2021, 30 (12): 126201. DOI: 10.1088/1674-1056/ac0524

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Flower-like tungsten disulfide (WS₂) with a diameter of 5-10 μm is prepared by chemical vapor deposition (CVD). Scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), Raman spectroscopy, and ultraviolet-visible (UV-vis) spectroscopy are used to characterize its morphological and optical properties, and its growth mechanism is discussed. The key factors for the formation of flower-like WS₂ are determined. Firstly, the cooling process causes the generation of nucleation dislocations, and then the "leaf" growth of flower-like WS₂ is achieved by increasing the temperature.

Nonlinear dynamical stability of gap solitons in Bose-Einstein condensate loaded in a deformed honeycomb optical lattice

Hongjuan Meng(蒙红娟), Yushan Zhou(周玉珊), Xueping Ren(任雪平), Xiaohuan Wan(万晓欢), Juan Zhang(张娟), Jing Wang(王静), Xiaobei Fan(樊小贝), Wenyuan Wang(王文元), and Yuren Shi(石玉仁)

Chin. Phys. B, 2021, 30 (12): 126701. DOI: 10.1088/1674-1056/ac009d

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We investigate the existence and dynamical stability of multipole gap solitons in Bose-Einstein condensate loaded in a deformed honeycomb optical lattice. Honeycomb lattices possess a unique band structure, the first and second bands intersect at a set of so-called Dirac points. Deformation can result in the merging and disappearance of the Dirac points, and support the gap solitons. We find that the two-dimensional honeycomb optical lattices admit multipole gap solitons. These multipoles can have their bright solitary structures being in-phase or out-of-phase. We also investigate the linear stabilities and nonlinear stabilities of these gap solitons. These results have applications of the localized structures in nonlinear optics, and may helpful for exploiting topological properties of a deformed lattice.

Moiré superlattice modulations in single-unit-cell FeTe films grown on NbSe₂ single crystals Suggested by editors

Han-Bin Deng(邓翰宾), Yuan Li(李渊), Zili Feng(冯子力), Jian-Yu Guan(关剑宇), Xin Yu(于鑫), Xiong Huang(黄雄), Rui-Zhe Liu(刘睿哲), Chang-Jiang Zhu(朱长江), Limin Liu(刘立民), Ying-Kai Sun(孙英开), Xi-Liang Peng(彭锡亮), Shuai-Shuai Li(李帅帅), Xin Du(杜鑫), Zheng Wang(王铮), Rui Wu(武睿), Jia-Xin Yin(殷嘉鑫), You-Guo Shi(石友国), and Han-Qing Mao(毛寒青)

Chin. Phys. B, 2021, 30 (12): 126801. DOI: 10.1088/1674-1056/ac0816

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Interface can be a fertile ground for exotic quantum states, including topological superconductivity, Majorana mode, fractal quantum Hall effect, unconventional superconductivity, Mott insulator, etc. Here we grow single-unit-cell (1UC) FeTe film on NbSe₂ single crystal by molecular beam epitaxy (MBE) and investigate the film in-situ with a home-made cryogenic scanning tunneling microscopy (STM) and non-contact atomic force microscopy (AFM) combined system. We find different stripe-like superlattice modulations on grown FeTe film with different misorientation angles with respect to NbSe₂ substrate. We show that these stripe-like superlattice modulations can be understood as moiré pattern forming between FeTe film and NbSe₂ substrate. Our results indicate that the interface between FeTe and NbSe₂ is atomically sharp. By STM-AFM combined measurement, we suggest that the moiré superlattice modulations have an electronic origin when the misorientation angle is relatively small (≤ 3°) and have structural relaxation when the misorientation angle is relatively large (≥ 10°).

Extended phase diagram of La_1-xCa_xMnO₃ by interfacial engineering ^Hot!

Kexuan Zhang(张可璇), Lili Qu(屈莉莉), Feng Jin(金锋), Guanyin Gao(高关胤), Enda Hua(华恩达), Zixun Zhang(张子璕), Lingfei Wang(王凌飞), and Wenbin Wu(吴文彬)

Chin. Phys. B, 2021, 30 (12): 126802. DOI: 10.1088/1674-1056/ac003e

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The interfacial enhanced ferromagnetism in maganite/ruthenate system is regarded as a promising path to broaden the potential of oxide-based electronic device applications. Here, we systematically studied the physical properties of LaLa_1-xCa_xMnO₃/SrRuO₃ superlattices and compared them with the LaLa_1-xCa_xMnO₃ thin films and bulk compounds. The LaLa_1-xCa_xMnO₃/SrRuO₃ superlattices exhibit significant enhancement of Curie temperature (T_C) beyond the corresponding thin films and bulks. Based on these results, we constructed an extended phase diagram of LaLa_1-xCa_xMnO₃ under interfacial engineering. We considered the interfacial charge transfer and structural proximity effects as the origin of the interface-induced high T_C. The structural characterizations revealed a pronounced increase of B-O-B bond angle, which could be the main driving force for the high T_C in the superlattices. Our work inspires a deeper understanding of the collective effects of interfacial charge transfer and structural proximity on the physical properties of oxide heterostructures.

Manipulating metal-insulator transitions of VO₂ films via embedding Ag nanonet arrays

Zhangyang Zhou(周章洋), Jia Yang(杨佳), Yi Liu(刘艺), Zhipeng Gao(高志鹏), Linhong Cao(曹林洪), Leiming Fang(房雷鸣), Hongliang He(贺红亮), and Zhengwei Xiong(熊政伟)

Chin. Phys. B, 2021, 30 (12): 126803. DOI: 10.1088/1674-1056/ac003a

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Manipulating metal-insulator transitions in strongly correlated materials is of great importance in condensed matter physics, with implications for both fundamental science and technology. Vanadium dioxide (VO₂), as an ideal model system, is metallic at high temperatures and shown a typical metal-insulator structural phase transition at 341 K from rutile structure to monoclinic structure. This behavior has been absorbed tons of attention for years. However, how to control this phase transition is still challenging and little studied. Here we demonstrated that to control the Ag nanonet arrays (NAs) in monoclinic VO₂(M) could be effective to adjust this metal-insulator transition. With the increase of Ag NAs volume fraction by reducing the template spheres size, the transition temperature (T_c) decreased from 68° to 51°. The mechanism of T_c decrease was revealed as:the carrier density increases through the increase of Ag NAs volume fraction, and more free electrons injected into the VO₂ films induced greater absorption energy at the internal nanometal-semiconductor junction. These results supply a new strategy to control the metal-insulator transitions in VO₂, which must be instructive for the other strongly correlated materials and important for applications.

Molecular beam epitaxy growth of monolayer hexagonal MnTe₂ on Si(111) substrate Suggested by editors

S Lu(卢帅), K Peng(彭坤), P D Wang(王鹏栋), A X Chen(陈爱喜), W Ren(任伟), X W Fang(方鑫伟), Y Wu(伍莹), Z Y Li(李治云), H F Li(李慧芳), F Y Cheng(程飞宇), K L Xiong(熊康林), J Y Yang(杨继勇), J Z Wang(王俊忠), S A Ding(丁孙安), Y P Jiang(蒋烨平), L Wang(王利), Q Li(李青), F S Li(李坊森), and L F Chi(迟力峰)

Chin. Phys. B, 2021, 30 (12): 126804. DOI: 10.1088/1674-1056/ac2e63

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Monolayer MnTe₂ stabilized as 1T structure has been theoretically predicted to be a two-dimensional (2D) ferromagnetic metal and can be tuned via strain engineering. There is no naturally van der Waals (vdW) layered MnTe₂ bulk, leaving mechanical exfoliation impossible to prepare monolayer MnTe₂. Herein, by means of molecular beam epitaxy (MBE), we successfully prepared monolayer hexagonal MnTe₂ on Si(111) under Te rich condition. Sharp reflection high-energy electron diffraction (RHEED) and low-energy electron diffraction (LEED) patterns suggest the monolayer is atomically flat without surface reconstruction. The valence state of Mn⁴⁺ and the atom ratio of ([Te]:[Mn]) further confirm the MnTe₂ compound. Scanning tunneling spectroscopy (STS) shows the hexagonal MnTe₂ monolayer is a semiconductor with a large bandgap of ~2.78 eV. The valence-band maximum (VBM) locates at the Γ point, as illustrated by angle-resolved photoemission spectroscopy (ARPES), below which three hole-type bands with parabolic dispersion can be identified. The successful synthesis of monolayer MnTe₂ film provides a new platform to investigate the 2D magnetism.

Electric and thermal transport properties of topological insulator candidate LiMgBi Suggested by editors

Hao OuYang(欧阳豪), Qing-Xin Dong(董庆新), Yi-Fei Huang(黄奕飞), Jun-Sen Xiang(项俊森), Li-Bo Zhang(张黎博), Chen-Sheng Li(李晨圣), Pei-Jie Sun(孙培杰), Zhi-An Ren(任治安), and Gen-Fu Chen(陈根富)

Chin. Phys. B, 2021, 30 (12): 127101. DOI: 10.1088/1674-1056/ac009f

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We report the transport properties of a topological insulator candidate, LiMgBi. The electric resistivity of the title compound exhibits a metal-to-semiconductor-like transition at around 160 K and tends to saturation below 50 K. At low temperatures, the magnetoresistance is up to ~260% at 9 T and a clear weak antilocalization effect is observed in the low magnetic-field region. The Hall measurement reveals that LiMgBi is a multiband system, where hole-type carriers (n_h~10¹⁸ cm^-3) play a major role in the transport process. Remarkably, LiMgBi possess a large Seebeck coefficient (~440 μV/K) and a moderate thermal conductivity at room temperature, which indicate that LiMgBi is a promising candidate in thermoelectric applications.

Comparison of resonant tunneling diodes grown on freestanding GaN substrates and sapphire substrates by plasma-assisted molecular-beam epitaxy

Xiang-Peng Zhou(周祥鹏), Hai-Bing Qiu(邱海兵), Wen-Xian Yang(杨文献), Shu-Long Lu(陆书龙), Xue Zhang(张雪), Shan Jin(金山), Xue-Fei Li(李雪飞), Li-Feng Bian(边历峰), and Hua Qin(秦华)

Chin. Phys. B, 2021, 30 (12): 127301. DOI: 10.1088/1674-1056/ac0525

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AlN/GaN resonant tunneling diodes (RTDs) were grown separately on freestanding GaN (FS-GaN) substrates and sapphire substrates by plasma-assisted molecular-beam epitaxy (PA-MBE). Room temperature negative differential resistance (NDR) was obtained under forward bias for the RTDs grown on FS-GaN substrates, with the peak current densities (J_p) of 175-700 kA/cm² and peak-to-valley current ratios (PVCRs) of 1.01-1.21. Two resonant peaks were also observed for some RTDs at room temperature. The effects of two types of substrates on epitaxy quality and device performance of GaN-based RTDs were firstly investigated systematically, showing that lower dislocation densities, flatter surface morphology, and steeper heterogeneous interfaces were the key factors to achieving NDR for RTDs.

Anomalous anisotropic magnetoresistance in single-crystalline Co/SrTiO₃(001) heterostructures

Shuang-Long Yang(杨双龙), De-Zheng Yang(杨德政), Yu Miao(缪宇), Cun-Xu Gao(高存绪), and De-Sheng Xue(薛德胜)

Chin. Phys. B, 2021, 30 (12): 127302. DOI: 10.1088/1674-1056/ac05ad

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The anisotropic magnetoresistances (AMRs) in single crystalline Co(6 nm)/SrTiO₃(001) heterostructures from 5 K to 300 K with the current direction setting along either Co[100] or Co[110] are investigated in this work. The anomalous (normal) AMR is observed below (above) 100 K. With the current along Co[100] direction, the AMR shows negative longitudinal and positive transverse magnetoresistances at T< 100 K, while the AMR is inverse with the current along Co[110]. Meanwhile, the amplitude ratio between Co[110] and Co[100] is observed to be as large as 29 at 100 K. A crystal symmetry-adapted model of AMR demonstrates that interplay between the non-crystalline component and crossed AMR component results in the anomalous AMR. Our results may reveal more intriguing magneto-transport behaviors of film on SrTiO₃ or other perovskite oxides.

Controllable and switchable chiral near-fields in symmetric graphene metasurfaces Suggested by editors

Li Hu(胡莉), Hongxia Dai(代洪霞), Fayin Cheng(程发银), and Yuxia Tang(唐裕霞)

Chin. Phys. B, 2021, 30 (12): 127303. DOI: 10.1088/1674-1056/ac2e5d

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A strong chiral near-field plays significant roles in the detection, separation and sensing of chiral molecules. In this paper, a simple and symmetric metasurface is proposed to generate strong chiral near-fields with both circularly polarized light and linearly polarized light illuminations in the mid-infrared region. Owing to the near-field interaction between plasmonic resonant modes of two nanosheets excited by circularly polarized light, there is a strong single-handed chiral near-field in the gap between the two graphene nanosheets and the maximum enhancement of the optical chirality could reach two orders of magnitude. As expected, the intensity and the response wavelength of the chiral near-fields could be controlled by the Fermi level and geometrical parameters of the graphene nanosheets, as well as the permittivity of the substrate. Meanwhile, based on the interaction between the incident field and scattered field, the one-handed chiral near-field in the gap also could be generated by the linearly polarized light excitation. For the two cases, the handedness of the chiral near-field could be switched by the polarized direction of the incident light. These results have potential opportunities for applications in molecular detection and sensing.

Angular dependence of vertical force and torque when magnetic dipole moves vertically above flat high-temperature superconductor

Yong Yang(杨勇), Shuai-Jie Yang(杨帅杰), Wen-Li Yang(杨文莉), and Yun-Yi Wu(吴云翼)

Chin. Phys. B, 2021, 30 (12): 127401. DOI: 10.1088/1674-1056/ac0697

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The interaction between a permanent magnet (PM) assumed as a magnetic dipole and a flat high-temperature superconductor (HTS) is calculated by the advanced frozen-image model. When the dipole vertically moves above the semi-infinite HTS, the general analytical expression of vertical force and that of torque are obtained for an arbitrary angle between the magnetization direction of the PM and the c axis of the HTS. The variations of the force and torque are analyzed for angle and vertical movements in both zero-field cooling (ZFC) condition and field cooling (FC) condition. It is found that the maximum vertical repulsive or attractive force has the positive or negative cosine relation with the angle. However, the maximum torque has the positive or negative sine relation. From the viewpoint of the rotational equilibrium, the orientation of the magnetic dipole with zero angle is deemed to be an unstable equilibrium point in ZFC, but the same orientation is considered as a stable equilibrium point in FC. In addition, both of the variation cycles of the maximum force and torque with the angle are π.

Dispersion of neutron spin resonance mode in Ba_0.67K_0.33Fe₂As₂

Tao Xie(谢涛), Chang Liu(刘畅), Tom Fennell, Uwe Stuhr, Shi-Liang Li(李世亮), and Hui-Qian Luo(罗会仟)

Chin. Phys. B, 2021, 30 (12): 127402. DOI: 10.1088/1674-1056/ac3651

Abstract ( 399 )

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We report an inelastic neutron scattering investigation on the spin resonance mode in the optimally hole-doped iron-based superconductor Ba_0.67K_0.33Fe₂As₂ with T_c=38.2 K. Although the resonance is nearly two-dimensional with peak energy E_R≈14 meV, it splits into two incommensurate peaks along the longitudinal direction ([H, 0, 0]) and shows an upward dispersion persisting to 26 meV. Such dispersion breaks through the limit of total superconducting gaps ∆_tot=|∆_k|+|∆_k+Q|(about 11-17 meV) on nested Fermi surfaces measured by high resolution angle resolved photoemission spectroscopy (ARPES). These results cannot be fully understood by the magnetic exciton scenario under s^±-pairing symmetry of superconductivity, and suggest that the spin resonance may not be restricted by the superconducting gaps in the multi-band systems.

Magnetic phase diagram of single-layer CrBr₃

Wei Jiang(江伟), Yue-Fei Hou(侯跃飞), Shujing Li(李淑静), Zhen-Guo Fu(付振国), and Ping Zhang(张平)

Chin. Phys. B, 2021, 30 (12): 127501. DOI: 10.1088/1674-1056/ac0043

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We theoretically provide a magnetic phase diagram for the single-layer (SL) CrBr₃, which could be effectively tuned by both strain engineering and charge doping in SL-CrBr₃. Through systematical first-principles calculations and Heisenberg model Hamiltonian simulations, three different magnetic phases in SL-CrBr₃, which are off-plane ferromagnetic, in-plane ferromagnetic and in-plane Néel-antiferromagnetic phases, are found in the strain and charge doping regimes we studied. Furthermore, our results show that higher order Heisenberg exchange parameters and anisotropy exchange parameters should be taken into account for accurately illustrating the magnetic phase transition in SL-CrBr₃. As a result, we find from the SpinW simulation that the Curie temperature is about T_c=38.4 K, which is well consistent with the experimental result 34 K[Nano Lett. 19 3138 (2019)]. The findings here may be confirmed in future experiments, and may be useful for the potential applications of SL-CrBr₃ in spintronics field.

Magnetic anisotropy manipulation and interfacial coupling in Sm₃Fe₅O₁₂ films and CoFe/Sm₃Fe₅O₁₂ heterostructures Suggested by editors

Lei Shen(沈磊), Guanjie Wu(武冠杰), Tao Sun(孙韬), Zhi Meng(孟智), Chun Zhou(周春), Wenyi Liu(刘文怡), Kang Qiu(邱康), Zongwei Ma(马宗伟), Haoliang Huang(黄浩亮), Yalin Lu(陆亚林), Zongzhi Zhang(张宗芝), and Zhigao Sheng(盛志高)

Chin. Phys. B, 2021, 30 (12): 127502. DOI: 10.1088/1674-1056/ac0040

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The magnetic anisotropy manipulation in the Sm₃Fe₅O₁₂ (SmIG) films and its effect on the interfacial spin coupling in the CoFe/SmIG heterostructures were studied carefully. By switching the orientation of the Gd₃Ga₅O₁₂ substrates from (111) to (001), the magnetic anisotropy of obtained SmIG films shifts from in-plane to out-of-plane. Similar results can also be obtained in the films on Gd₃Sc₂Ga₃O₁₂ substrates, which identifies the universality of such orientation-induced magnetic anisotropy switching. Additionally, the interfacial spin coupling and magnetic anisotropy switching effect on the spin wave in CoFe/SmIG magnetic heterojunctions have also been explored by utilizing the time-resolved magneto-optical Kerr effect technique. It is intriguing to find that both the frequency and effective damping factor of spin precession in CoFe/SmIG heterojunctions can be manipulated by the magnetic anisotropy switching of SmIG films. These findings not only provide a route for the perpendicular magnetic anisotropy acquisition but also give a further path for spin manipulation in magnetic films and heterojunctions.

Insight into influence of thermodynamic coefficients on transient negative capacitance in Zr-doped HfO₂ ferroelectric capacitors

Yuan-Yuan Zhang(张元元), Xiao-Qing Sun(孙晓清), Jun-Shuai Chai(柴俊帅), Hao Xu(徐昊), Xue-Li Ma(马雪丽), Jin-Juan Xiang(项金娟), Kai Han(韩锴), Xiao-Lei Wang(王晓磊), and Wen-Wu Wang(王文武)

Chin. Phys. B, 2021, 30 (12): 127701. DOI: 10.1088/1674-1056/ac01c4

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We study the influence of the thermodynamic coefficients on transient negative capacitance for the Zr-doped HfO₂ (HZO) ferroelectric capacitors by the theoretical simulation based on the Landau-Khalatnikov (L-K) theory and experimental measurement of electrical properties in the resistor-ferroelectric capacitor (R-FEC) circuit. Our results show that the thermodynamic coefficients α, β and γ also play a key role for the transient NC effect besides the viscosity coefficient and series resistor. Moreover, the smaller coefficients α and β, the more significant the transient NC effect. In addition, we also find that the thermodynamic process of transient NC does not obey the generally accepted viewpoint of Gibbs free energy minimization.

Synthesis of ternary compound in H-S-Se system at high pressures Suggested by editors

Xiao Zhang(张晓)

Chin. Phys. B, 2021, 30 (12): 127801. DOI: 10.1088/1674-1056/abf7aa

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The chemical reaction products of elemental sulfur (S), selenium (Se), and molecular hydrogen (H₂) at high pressures and room temperature are probed by Raman spectroscopy. Two known compounds H₂S and H₂Se can be synthesized after laser heating at pressures lower than 1 GPa. Under further compression at room temperature, an H₂S-H₂Se and an H₂S-H₂Se-H₂ van der Waals compounds are synthesized at 4 GPa and 6 GPa, respectively. The later is of guest-host structure and can be identified as (H₂S)_x(H₂Se)_(2-x)H₂. It can be maintained up to 37 GPa at least, and the stability of its H₂Se molecules is extended:the H-Se stretching mode can be detected at least to 36 GPa but disappears at 22 GPa in (H₂Se)₂H₂. The pressure dependence of S-H and Se-H stretching modes of this ternary compound is in line with that of (H₂S)₂H₂ and (H₂Se)₂H₂, respectively. However, its hydrogen subsystem only shows the relevance to (H₂S)₂H₂, indicating that this ternary compound can be viewed as H₂Se-replaced partial H₂S of (H₂S)₂H₂.

Tuning energy transfer efficiency in quantum dots mixture by controling donor/acceptor ratio ^Hot!

Chang Liu(刘畅), Jing Liang(梁晶), Fangfang Wang(王芳芳), Chaojie Ma(马超杰), Kehai Liu(刘科海), Can Liu(刘灿), Hao Hong(洪浩), Huaibin Shen(申怀彬), Kaihui Liu(刘开辉), and Enge Wang(王恩哥)

Chin. Phys. B, 2021, 30 (12): 127802. DOI: 10.1088/1674-1056/ac29b2

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Improving the emission performance of colloidal quantum dots (QDs) is of paramount importance for their applications on light-emitting diodes (LEDs), displays and lasers. A highly promising approach is to tune the carrier recombination channels and lifetime by exploiting the energy transfer process. However, to achieve this precise emission optimization, quantitative modulation on energy transfer efficiency is highly desirable but still challenging. Here, we demonstrate a convenient approach to realize tunable energy transfer efficiency by forming QDs mixture with controllable donor/acceptor (D/A) ratio. With the mixing ratio ranging from 16/1 to 1/16, the energy transfer efficiency could be effectively tuned from near zero to ~70%. For the high mixing ratio of 16/1, acceptors obtain adequate energy supplied by closely surrounding donors, leading to~2.4-fold PL enhancement. While for the low mixing ratio, the ultrafast and efficient energy extraction process directly suppresses the multi-exciton and Auger recombination in the donor, bringing about a higher threshold. The facile modulation of emission performance by controllably designed mixing ratio and quantitatively tunable energy transfer efficiency will facilitate QD-based optoelectronic and photovoltaic applications.

Tunable wide-angle multi-band mid-infrared linear-to-linear polarization converter based on a graphene metasurface

Lan-Lan Zhang(张兰兰), Ping Li(李萍), and Xiao-Wei Song(宋霄薇)

Chin. Phys. B, 2021, 30 (12): 127803. DOI: 10.1088/1674-1056/ac0cdd

Abstract ( 418 )

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We present a high-efficiency tunable wide-angle multi-band reflective linear-to-linear (LTL) polarization converter, which is composed of an array of two L-shaped graphene patches with different sizes. In the mid-infrared region, the proposed converter can transform x-polarized wave into y-polarized wave at four resonant frequencies. The polarization conversion ratios of the four bands reach 94.4%, 92.7%, 99.3%, and 93.1%, respectively. By carefully choosing the geometric parameter, triple-band LTL polarization conversion can also be realized. The three polarization conversion ratios reach 91.50%, 99.20%, and 97.22%, respectively. The influence of incident angle on the performances of the LTL polarization converter is investigated, and it is found that our polarization converter shows the angle insensitivity. Also, the dynamically tunable properties of the proposed polarization converter are numerically studied by changing Fermi energy. All the simulation results are conducted by finite element method.

Topological Dirac surface states in ternary compounds GeBi₂Te₄, SnBi₂Te₄ and Sn_0.571Bi_2.286Se₄ ^Hot!

Yunlong Li(李云龙), Chaozhi Huang(黄超之), Guohua Wang(王国华), Jiayuan Hu(胡佳元), Shaofeng Duan(段绍峰), Chenhang Xu(徐晨航), Qi Lu(卢琦), Qiang Jing(景强), Wentao Zhang(张文涛), and Dong Qian(钱冬)

Chin. Phys. B, 2021, 30 (12): 127901. DOI: 10.1088/1674-1056/ac2b92

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Using high-resolution angle-resolved and time-resolved photoemission spectroscopy, we have studied the low-energy band structures in occupied and unoccupied states of three ternary compounds GeBi₂Te₄, SnBi₂Te₄ and Sn_0.571Bi_2.286Se₄ near the Fermi level. In previously confirmed topological insulator GeBi₂Te₄ compounds, we confirmed the existence of the Dirac surface state and found that the bulk energy gap is much larger than that in the first-principles calculations. In SnBi₂Te₄ compounds, the Dirac surface state was observed, consistent with the first-principles calculations, indicating that it is a topological insulator. The experimental detected bulk gap is a little bit larger than that in calculations. In Sn_0.571Bi_2.286Se₄ compounds, our measurements suggest that this nonstoichiometric compound is a topological insulator although the stoichiometric SnBi₂Se₄ compound was proposed to be topological trivial.

Thermal and mechanical properties and micro-mechanism of SiO₂/epoxy nanodielectrics

Tian-Yu Wang(王天宇), Gui-Xin Zhang(张贵新), and Da-Yu Li(李大雨)

Chin. Phys. B, 2021, 30 (12): 128101. DOI: 10.1088/1674-1056/abff2c

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In addition to electrical insulation properties, the thermal properties of nanodielectrics, such as glass transition temperature, thermal expansion coefficients, thermal conductivity, and mechanical properties, including Young's modulus, bulk modulus, and shear modulus, are also very important. This paper describes the molecular dynamics simulations of epoxy resin doped with SiO₂ nanoparticles and with SiO₂ nanoparticles that have been surface grafted with hexamethyldisilazane (HMDS) at 10% and 20% grafting rates. The results show that surface grafting can improve certain thermal and mechanical properties of the system. Our analysis indicates that the improved thermal performance occurs because the formation of thermal chains becomes easier after the surface grafting treatment. The improved mechanical properties originate from two causes. First, doping with SiO₂ nanoparticles inhibits the degree of movement of molecular chains in the system. Second, the surface grafting treatment weakens the molecular repulsion between SiO₂ and epoxy resin, and the van der Waals excluded region becomes thinner. Thus, the compatibility between SiO₂ nanoparticles and polymers is improved by the grafting treatment. The analysis method and conclusions in this paper provide guidance and reference for the future studies of the thermal and mechanical properties of nanodielectrics.

Distribution of donor states on the surfaceof AlGaN/GaN heterostructures

Yue-Bo Liu(柳月波), Hong-Hui Liu(刘红辉), Jun-Yu Shen(沈俊宇), Wan-Qing Yao(姚婉青), Feng-Ge Wang(王风格), Yuan Ren(任远), Min-Jie Zhang(张敏杰), Zhi-Sheng Wu(吴志盛), Yang Liu(刘扬), and Bai-Jun Zhang(张佰君)

Chin. Phys. B, 2021, 30 (12): 128102. DOI: 10.1088/1674-1056/ac0792

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The uniform distribution model of the surface donor states in AlGaN/GaN heterostructures has been widely used in the theoretical calculation. A common and a triple-channel AlGaN/GaN heterostructure Schottky barrier diodes have been fabricated to verify the models, but the calculation results show the uniform distribution model can not provide enough electrons to form three separate 2DEGs in the triple-channel AlGaN/GaN heterostructure. Our experiments indicate the uniform distribution model is not quite right, especially for the multiple-channel AlGaN/GaN heterostructures. Besides, it is found the exponential distribution model possibly matches the actual distribution of the surface donor states better, which allows the 2DEG to form in each channel structure during the calculation. The exponential distribution model would be helpful in the research field.

Morphological effect on electrochemical performance of nanostructural CrN

Zhengwei Xiong(熊政伟), Xuemei An(安雪梅), Qian Liu(刘倩), Jiayi Zhu(朱家艺), Xiaoqiang Zhang(张小强), Chenchun Hao(郝辰春), Qiang Yang(羊强), Zhipeng Gao(高志鹏), and Meng Zhang(张盟)

Chin. Phys. B, 2021, 30 (12): 128201. DOI: 10.1088/1674-1056/ac1f07

Abstract ( 306 )

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Size and morphology are critical factors in determining the electrochemical performance of the supercapacitor materials, due to the manifestation of the nanosize effect. Herein, different nanostructures of the CrN material are prepared by the combination of a thermal-nitridation process and a template technique. High-temperature nitridation could not only transform the hexagonal Cr₂O₃ into cubic CrN, but also keep the template morphology barely unchanged. The obtained CrN nanostructures, including (i) hierarchical microspheres assembled by nanoparticles, (ii) microlayers, and (iii) nanoparticles, are studied for the electrochemical supercapacitor. The CrN microspheres show the best specific capacitance (213.2 F/g), cyclic stability (capacitance retention rate of 96% after 5000 cycles in 1-mol/L KOH solution), high energy density (28.9 Wh/kg), and power density (443.4 W/kg), comparing with the other two nanostructures. Based on the impedance spectroscopy and nitrogen adsorption analysis, it is revealed that the enhancement arised mainly from a high-conductance and specific surface area of CrN microspheres. This work presents a general strategy of fabricating controllable CrN nanostructures to achieve the enhanced supercapacitor performance.

Excellent thermoelectric performance predicted in Sb₂Te with natural superlattice structure

Pei Zhang(张培), Tao Ouyang(欧阳滔), Chao Tang(唐超), Chaoyu He(何朝宇), Jin Li(李金), Chunxiao Zhang(张春小), and Jianxin Zhong(钟建新)

Chin. Phys. B, 2021, 30 (12): 128401. DOI: 10.1088/1674-1056/ac012c

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Using first-principles calculations combined with the Boltzmann transport theory, we explore the thermoelectric properties of natural superlattice (SL) structure Sb₂Te. The results show that n-type Sb₂Te possesses larger Seebeck coefficient of 249.59 (318.87) μV/K than p-type Sb₂Te of 219.85 (210.38) μV/K and low lattice thermal conductivity of 1.25 (0.21) W/mK along the in-plane (out-of-plane) direction at 300 K. The excellent electron transport performance is mainly attributed to steeper density of state around the bottom of conduction band. The ultralow lattice thermal conductivity of Sb₂Te is mainly caused by low phonon group velocity and strong anharmonicity. Further analysis shows that the decrease of group velocity comes from flatter dispersion curves which are contributed by the Brillouin-zone folding. The strong anharmonicity is mainly due to the presence of lone-pair electrons in Sb₂Te. Combining such a high Seebeck coefficient with the low lattice thermal conductivity, maximum n-type thermoelectric figure of merit (ZT) of 1.46 and 1.38 could be achieved along the in-plane and out-of-plane directions at room temperature, which is higher than the reported values of Sb₂Te₃. The findings presented here provide insight into the transport property of Sb₂Te and highlight potential applications of thermoelectric materials at room temperature.

A review on the design of ternary logic circuits

Xiao-Yuan Wang(王晓媛), Chuan-Tao Dong(董传涛), Zhi-Ru Wu(吴志茹), and Zhi-Qun Cheng(程知群)

Chin. Phys. B, 2021, 30 (12): 128402. DOI: 10.1088/1674-1056/ac248b

Abstract ( 533 )

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A multi-valued logic system is a promising alternative to traditional binary logic because it can reduce the complexity, power consumption, and area of circuit implementation. This article briefly summarizes the development of ternary logic and its advantages in digital logic circuits. The schemes, characteristics, and application of ternary logic circuits based on CMOS, CNTFET, memristor, and other devices and processes are reviewed in this paper, providing some reference for the further research and development of ternary logic circuits.

A double quantum dot defined by top gates in a single crystalline InSb nanosheet ^Hot!

Yuanjie Chen(陈元杰), Shaoyun Huang(黄少云), Jingwei Mu(慕经纬), Dong Pan(潘东), Jianhua Zhao(赵建华), and Hong-Qi Xu(徐洪起)

Chin. Phys. B, 2021, 30 (12): 128501. DOI: 10.1088/1674-1056/abff2e

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We report on the transport study of a double quantum dot (DQD) device made from a freestanding, single crystalline InSb nanosheet. The freestanding nanosheet is grown by molecular beam epitaxy and the DQD is defined by the top gate technique. Through the transport measurements, we demonstrate how a single quantum dot (QD) and a DQD can be defined in an InSb nanosheet by tuning voltages applied to the top gates. We also measure the charge stability diagrams of the DQD and show that the charge states and the inter-dot coupling between the two individual QDs in the DQD can be efficiently regulated by the top gates. Numerical simulations for the potential profile and charge density distribution in the DQD have been performed and the results support the experimental findings and provide a better understanding of fabrication and transport characteristics of the DQD in the InSb nanosheet. The achieved DQD in the two-dimensional InSb nanosheet possesses pronounced benefits in lateral scaling and can thus serve as a new building block for the developments of quantum computation and quantum simulation technologies.

Fabrication of Josephson parameter amplifier and its applicationin squeezing vacuum fluctuations Suggested by editors

Pengtao Song(宋鹏涛), Xueyi Guo(郭学仪), Kai Xu(许凯), Xiaohui Song(宋小会), Zhan Wang(王战), Zhongcheng Xiang(相忠诚), Hekang Li(李贺康), Luhong Su(苏鹭红), Yirong Jin(金贻荣), and Dongning Zheng(郑东宁)

Chin. Phys. B, 2021, 30 (12): 128502. DOI: 10.1088/1674-1056/ac0796

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Josephson parameter amplifier (JPA) is a microwave signal amplifier device with near-quantum-limit-noise performance. It has important applications in scientific research fields such as quantum computing and dark matter detection. This work reports the fabrication and characterization of broadband JPA devices and their applications in multi-qubit readout and squeezing of vacuum state. We use a process in which transmission lines and electrodes are made of niobium thin film and aluminum Josephson junctions are made by Dolan bridge technique. We believe this process is more convenient than the process we used previously. The whole production process adopts electron beam lithography technology to ensure high structural resolution. The test result shows that the gain value of the manufactured JPA can exceed 15 dB, and the amplification bandwidth is about 400 MHz. The noise temperature is about 400 mK at the working frequency of 6.2 GHz. The devices have been successfully used in experiments involving superconducting multi-qubit quantum processors. Furthermore, the device is applied to squeeze vacuum fluctuations and a squeezing level of 1.635 dB is achieved.

Fitness of others' evaluation effect promotes cooperation in spatial public goods game

Jian-Wei Wang(王建伟), Rong Wang(王蓉), and Feng-Yuan Yu(于逢源)

Chin. Phys. B, 2021, 30 (12): 128701. DOI: 10.1088/1674-1056/ac05ae

Abstract ( 371 )

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Payoff-driven strategy updating rule has always been adopted as a classic mechanism, but up to now, there have been a great many of researches on considering other forms of strategy updating rules, among which pursuing high fitness is one of the most direct and conventional motivations in the decision-making using game theory. But there are few or no researches on fitness from the perspective of others' evaluation. In view of this, we propose a new model in which the evaluation effect with fitness-driven strategy updating rule is taken into consideration, and introduce an evaluation coefficient to present the degree of others' evaluation on individual's behavior. The cooperative individuals can get positive evaluation, otherwise defective individuals get negative evaluation, and the degree of evaluation is related to the number of neighbors who have the same strategy of individual. Through numerical simulation, we find that the evaluation effect of others can enhance the network reciprocity, thus promoting the cooperation. For a strong dilemma, the higher evaluation coefficient can greatly weaken the cooperation dilemma; for a weak one, the higher evaluation coefficient can make cooperator clusters spread faster, however, there is no significant difference in the level of cooperation in the final stable state among different evaluation coefficients. The cooperation becomes more flourish as the number of fitness-driven individuals increases, when all individuals adopt fitness-driven strategy updating rule, the cooperators can quickly occupy the whole population. Besides, we demonstrate the robustness of the results on the WS small-world network, ER random network, and BA scale-free network.

Continuous non-autonomous memristive Rulkov model with extreme multistability

Quan Xu(徐权), Tong Liu(刘通), Cheng-Tao Feng(冯成涛), Han Bao(包涵), Hua-Gan Wu(武花干), and Bo-Cheng Bao(包伯成)

Chin. Phys. B, 2021, 30 (12): 128702. DOI: 10.1088/1674-1056/ac2f30

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Based on the two-dimensional (2D) discrete Rulkov model that is used to describe neuron dynamics, this paper presents a continuous non-autonomous memristive Rulkov model. The effects of electromagnetic induction and external stimulus are simultaneously considered herein. The electromagnetic induction flow is imitated by the generated current from a flux-controlled memristor and the external stimulus is injected using a sinusoidal current. Thus, the presented model possesses a line equilibrium set evolving over the time. The equilibrium set and their stability distributions are numerically simulated and qualitatively analyzed. Afterwards, numerical simulations are executed to explore the dynamical behaviors associated to the electromagnetic induction, external stimulus, and initial conditions. Interestingly, the initial conditions dependent extreme multistability is elaborately disclosed in the continuous non-autonomous memristive Rulkov model. Furthermore, an analog circuit of the proposed model is implemented, upon which the hardware experiment is executed to verify the numerically simulated extreme multistability. The extreme multistability is numerically revealed and experimentally confirmed in this paper, which can widen the future engineering employment of the Rulkov model.

Discontinuous and continuous transitions of collective behaviors in living systems ^Hot!

Xu Li(李旭), Tingting Xue(薛婷婷), Yu Sun(孙宇), Jingfang Fan(樊京芳), Hui Li(李辉), Maoxin Liu(刘卯鑫), Zhangang Han(韩战钢), Zengru Di(狄增如), and Xiaosong Chen(陈晓松)

Chin. Phys. B, 2021, 30 (12): 128703. DOI: 10.1088/1674-1056/ac3c3f

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Living systems are full of astonishing diversity and complexity of life. Despite differences in the length scales and cognitive abilities of these systems, collective motion of large groups of individuals can emerge. It is of great importance to seek for the fundamental principles of collective motion, such as phase transitions and their natures. Via an eigen microstate approach, we have found a discontinuous transition of density and a continuous transition of velocity in the Vicsek models of collective motion, which are identified by the finite-size scaling form of order-parameter. At strong noise, living systems behave like gas. With the decrease of noise, the interactions between the particles of a living system become stronger and make them come closer. The living system experiences then a discontinuous gas-liquid like transition of density. The even stronger interactions at smaller noise make the velocity directions of the particles become ordered and there is a continuous phase transition of collective motion in addition.

Evolution mechanism of Weibo top news competition Suggested by editors

Fuzhong Nian(年福忠), Jingzhou Li(李经洲), and Xin Guo(郭鑫)

Chin. Phys. B, 2021, 30 (12): 128901. DOI: 10.1088/1674-1056/ac05ac

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In a certain period, some news will compete for the top news to gain the most attention and influence, and more news will be submerged in the ocean of news and become mediocre. This article deeply studies the evolution process and competition mechanism of the dissemination of Weibo news. In this paper, we innovatively propose a pre-processing scheme for traditional small-world networks and scale-free networks and divide nodes into three roles:fans, passersby, and anti-fans. The competition mechanism of Weibo top news is defined from the aspects of node role and node aggregation degree. A network evolution model is established based on the competition mechanism. The propagation characteristics of the network evolution model are deeply analyzed, and simulation experiments are performed on the small-world network and the scale-free network. Finally, the validity and rationality of the new model are verified through comparative experiments, and a feasible scheme for the propagation of top news on Weibo is given.

Proton loss of inner radiation belt during geomagnetic storm of 2018 based on CSES satellite observation

Zhen-Xia Zhang(张振霞), Xu-Hui Shen(申旭辉), Xin-Qiao Li(李新乔), and Yong-Fu Wang(王永福)

Chin. Phys. B, 2021, 30 (12): 129401. DOI: 10.1088/1674-1056/ac1f00

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The proton distribution in inner radiation belt is often affected by strong geomagnetic storm disturbance. Based on the data of the sun-synchronous CSES satellite, which carries with several high energy particle payloads and was launched in February 2018, we analyzed the extensive proton variations in the inner radiation belt in a wide energy range of 2 MeV-220 MeV during 2018 major geomagnetic storm. The result indicates that the loss mechanism of protons was energy dependence which is consistent with some previous studies. For protons at low energy 2 MeV-20 MeV, the fluxes were decreased during main phase of the storm and did not come back quickly during the recovery phase, which is likely to be caused by Coulomb collision due to neutral atmosphere density variation. At higher energy 30 MeV-100 MeV, it was confirmed that the magnetic field line curvature scattering plays a significant role in the proton loss phenomenon during this storm. At highest energies > 100 MeV, the fluxes of protons kept a stable level and did not exhibit a significant loss during this storm.

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